ENH: Bring the core anchoring options up to par with its ligand-based counterpart

[BvB93]

Closes #210

As of the moment there still two features that should be implemented at some point:

• Specifying multiple anchors for the core.
• Figuring out an appropiate set of core.anchor.kind options.

Examples

# Note that bonds *must* be present in the core in order to match the specified group
optional:
    core:
        anchor:
            group: "[H]Cl"  # Remove HCl and attach at previous Cl position
            group_idx: 1
            remove: [0, 1]

[codecov]

Codecov Report

Merging #211 (1ddc0a3) into master (23a3466) will increase coverage by 0.27%.
The diff coverage is 86.59%.

@@            Coverage Diff             @@
##           master     #211      +/-   ##
==========================================
+ Coverage   67.43%   67.71%   +0.27%     
==========================================
  Files          48       49       +1     
  Lines        3900     3958      +58     
  Branches      699      714      +15     
==========================================
+ Hits         2630     2680      +50     
- Misses       1114     1118       +4     
- Partials      156      160       +4     

Impacted Files	Coverage Δ
CAT/data_handling/validation_schemas.py	92.07% <ø> (ø)
CAT/attachment/core_anchoring.py	75.00% <75.00%> (ø)
CAT/base.py	67.32% <100.00%> (-1.29%)	⬇️
CAT/data_handling/anchor_parsing.py	100.00% <100.00%> (ø)
CAT/data_handling/validate_input.py	65.13% <100.00%> (+0.32%)	⬆️
CAT/data_handling/mol_import.py	73.52% <0.00%> (+0.42%)	⬆️

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