Merge branch 'marc/58-support-spack-and-others' into 'master'

Resolve "Support Spack and others"

### Usage information

Download Spack and setup environment:
```bash
git clone -c feature.manyFiles=true https://github.com/spack/spack.git
. spack/share/spack/setup-env.sh
```
Add GekkoFS Spack repository to Spack:
```bash
spack repo add gekkofs/scripts/spack
```
Check that Spack can find GekkoFS:
```
spack info gekkofs
```
Install GekkoFS (and run optional tests). Check `spack info gekkofs` for available option and versions:
```bash
spack install gekkofs
# for installing tests dependencies and running tests
spack install -v --test=root gekkofs +tests
```
Load GekkoFS into environment:
```
spack load gekkofs
```
If you want to use the latest developer branch of GekkoFS:
```
spack install gekkofs@latest
```
The default is using version 0.9.1 the last stable release.

### TODO

- [x] Base Spack functionality, versions, and configuration support
- [x] Documentation
- [x] Advanced functionality, more detailed configuration support, e.g., Parallax and Prometheus
- [x] More easy way to get path to client library
- [x] Add GekkoFS client wrapper for `LD_PRELOAD`
- [ ] Add final version to main Spack repository if possible. (Not possible right now as it not clear how 3rd party libraries should be treated.





















Closes #58

Closes #58

See merge request !137