Make cclib an optional dependency

[schneiderfelipe]

Initially, I wanted to exclusively use cclib for reading logfiles. Unfortunately, this was not possible and I patched the cclib output when reading ORCA logfiles. As such, cclib might become an optional dependency for reading logfiles other than ORCA generated ones.

When doing this, I might need to test some logfiles from at least one open source package (NWChem or Psi4 are fine).

[schneiderfelipe]

In the current state of things, cclib is necessary (and extremely useful). It will probably read xtb files in the future (cclib/cclib#789 and cclib/cclib#808), making part of our "add-on" parser unnecessary. Closing.