This MR enables the generation of code coverage reports for the source code. More specifically, it introduces the following changes:

- Adds the `gkfs-code-coverage.cmake` CMake script under `{PROJECT_ROOT}/CMake/` and includes it in the project's main `CMakeLists.txt`. This script defines the `GKFS_ENABLE_CODE_COVERAGE` option to control whether code coverage should be enabled when building, as well as the `target_code_coverage()` function to ensure that the appropriate flags are added to a CMake target.
- Adds `gcovr` to GekkoFS docker build image.
- Adds a `coverage.sh` script under `{PROJECT_ROOT}/scripts/ci` that provides both a `capture` and a `merge` work modes. This script abstracts away the direct usage of `gcovr` and allows the appropriate generation of coverage reports when running tests **in parallel** in the CI. The script relies on a `.coverage-exclusions` file (also under `{PROJECT_ROOT}/scripts/ci`) to control which paths should be excluded from coverage reporting (e.g. `/usr/include/.*`).
- Adds a new `report` stage in the CI pipeline that upon completion generates artifacts both for a Cobertura XML and a HTML code coverage reports.

See merge request !77

Resolve "Add path resolution tests"

Closes #108

See merge request !48

Optimization of create, stat, and remove operations. The following changes have been made:

- `gkfs_open()` logic refactored.
- create: Previously a stat RPC was send before each create to make sure the file doesn't exist. This logic is now implicit in the create operation on the daemon side.
- `get_metadata()` was used on the client for all stat operation, creating a `shared_ptr` for the `Metadata` object in the process which was not needed. A new function `get_metadata_attr()` was created to only return the metadata binary string, which will only create the metadata object if actually required.
- Remove logic was separated into two operations: `remove_metadata()` and `remove_data()`. 
- `gkfs::config::metadata::implicit_data_removal` setting: If `true`, will remove data on the same node during the `remove_metadata()` RPC. This is mainly an optimization, but will be useful for future asynchronous removal implementations.

Previously, the code path looked like this:
1. `stat()` to get `size` and `mode`.
2. Use these fields to determine if data needs to be removed or just metadata.
3. `remove()` is called which, first, sends a single RPC to the daemon with the metadata. Afterwards, data is removed.
4. The daemon used one handler for both cases.

It now looks like this: 
1. `remove()` is called which, first, sends a single RPC to the daemon with the metadata. Before, removing the metadata, the daemon fetches `mode` and `size`. If `implicit_data_removal` is set as a configuration, the data is removed in this RPC as well. `mode` and `size` are returned to the client.
2. The client determines if data needs to be removed as well.
3. If yes, sends a `remove_data()` RPC as it was previously.

Depends on !66 and !74.

Closes #94

See merge request !60

Removes 1 stat RPC per remove operation

Resolve "Don't use popen directly in integration tests"

Closes #141

See merge request hpc/gekkofs!80

Resolve "Unit tests not found"

Closes #140

See merge request !79

Resolve "Make chunk computations constexpr"

This MR implements the following changes:

- All arithmetic operations based on block sizes are now constexpr.
- Add several new functions, also constexpr: is_aligned(),
is_power_of_2().  - Reimplement log2() to make it faster based on
compiler builtins.  - Rename functions to avoid depending on the concept
of chunk (in preparation for more chunk-agnostic code).  - Add
exhaustive tests for all these functions and enable them in the CI.
- Rename the gkfs::util namespace to gkfs::utils.  - Create the
gkfs::utils::arithmetic namespace for all these functions, as well as
a convenience library/target in CMake for easier testing.

Fixes #137
Closes #135

See merge request !75