Check availability of theseus/mmligner executables

Description

We don't have Windows versions for theseus and mmligner as of now, thus currently no working Windows opencadd version.

Our workaround suggested by @jaimergp: Add safety checks for these packages (incl. informative text for our users). With such a workaround we will be able to offer this package for Windows, too (without functionality for the bits that need theseus and mmligner though). If these packages will be available in the future, the respective bits in opencadd will work out-of-the-box, too.

More background details:

Our package is labeled as noarch, which means we actually build it on linux (could be any other platform, but condaforge chose that one) and then hope it works in other OSs as well
To be noarch our package needs to be pure python, no compiled extensions, and no per-platform dependencies
We do need per-platform dependencies because some of those used in opencadd are not on windows
We are ignoring that and adding checks in our code as a workaround
When the dependencies become available, it will work magically in opencadd

Todos

[x] Add .safety_checks() method to BaseAligner but implement code in respective classes, see below
[x] Add _safety_checks() implementation to TheseusAligner and MMLignerAligner:
```
from distutils.spawn import find_executable

def align_with_theseus():
    theseus = find_executable("theseus")
    is theseus is None:
        raise Error("theseus cannot be located. Is it installed?")
    # proceed normally 
```
- [x] Add _safety_checks() to [MDAnalysisAligner]; only added for consistency across engines; simply passes (no executable check necessary since conda package available for all OS)
- [x] Call these methods in BaseAligner.calculate method

Questions

None.

Status

[x] Ready to go

volkamerlab / opencadd