get rid of `--toolchain` flag

See merge request mpsd-cs/mpsd-software!61

renamed "args.toolchains" to "args.toolchain" because that looks better in the
help output:

```
./mpsd-software.py install --help
usage: mpsd-software.py install [-h] [--enable-build-cache] release toolchain [toolchain ...]
```

is better than

```
./mpsd-software.py install --help
usage: mpsd-software.py install [-h] [--enable-build-cache] release toolchains [toolchains ...]
```

Move top level log

See merge request mpsd-cs/mpsd-software-environments!59

Catch potential user error

See merge request mpsd-cs/mpsd-software-environments!56

just tried this, so we might as well document the behaviour

See merge request mpsd-cs/mpsd-software-environments!58

because I wanted to know if package sets are included in the return value

- if passed to 'available', this creates a long and not very helpful message
  that lists all branches.
- if we catch it here, we avoid that.

improve wording of help strings and outputs

See merge request mpsd-cs/mpsd-software-environments!55

amongst other things explain why 'available' is called 'available'

fixes bug !49

See merge request mpsd-cs/mpsd-software-environments!54

Can easily re-add when the feature comes. The argument is not used in the
function yet at all. It is not even clear this is the right place to clear the
environment before we call 'install' again.

(Function takes one argument less now after
https://gitlab.gwdg.de/mpsd-cs/mpsd-software-environments/-/merge_requests/49)

neither `start-new` nor `remove` should be exposed to user at the moment

See merge request mpsd-cs/mpsd-software-environments!50

fix issue #9

Closes #9

See merge request mpsd-cs/mpsd-software-environments!52

# Conflicts:
#   mpsd-software.py

If toolchain cannot be found, point to `available` command.

See merge request mpsd-cs/mpsd-software-environments!51

reinstallation functionality missing - let's hide it from the users

See merge request mpsd-cs/mpsd-software-environments!49

(https://gitlab.gwdg.de/mpsd-cs/mpsd-software-environments/-/issues/9)

as they do not work; their existence will only cause confusion

prepare: update spack-environments repo if it exists already

See merge request mpsd-cs/mpsd-software-environments!48

Status command : add module commands for each microarch

See merge request mpsd-cs/mpsd-software-environments!46

Provide --version functionality

See merge request mpsd-cs/mpsd-software-environments!44