ENH: Add basic initial support for periodic ADF calculations

[BvB93]

Xref https://github.com/nlesc-nano/auto-FOX/issues/202 and https://github.com/nlesc-nano/auto-FOX/issues/163.

Currently there are still two limitations for periodic ADF calculations:

• The system's lattice must be cuboid, e.g. hexagonal lattices are not supported yet.
• The r_max parameter must be finite set to a finite value, e.g. the default of 8 Angstrom.

[codecov[bot]]

Codecov Report

Merging #220 (fed3631) into master (986bb86) will increase coverage by 0.11%. The diff coverage is 56.79%.

@@            Coverage Diff             @@
##           master     #220      +/-   ##
==========================================
+ Coverage   76.23%   76.34%   +0.11%     
==========================================
  Files          61       61              
  Lines        6442     6443       +1     
  Branches     1110     1117       +7     
==========================================
+ Hits         4911     4919       +8     
+ Misses       1243     1232      -11     
- Partials      288      292       +4     

Impacted Files	Coverage Δ
FOX/classes/multi_mol.py	75.83% <54.54%> (+2.14%)	:arrow_up:
FOX/functions/adf.py	68.75% <58.33%> (-31.25%)	:arrow_down:
FOX/io/read_psf.py	81.88% <0.00%> (ø)

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