Add compatibility layer for integration with Vasp

[awvwgk]

Add updated subdftd4.F and mocked Vasp modules to test compilation.

Closes #85
Closes dftd4/dftd4_vasp#3

[codecov]

Codecov Report

Merging #113 (e0fd7c4) into main (e66e1e2) will decrease coverage by 0.87%.
The diff coverage is 0.00%.

❗ Current head e0fd7c4 differs from pull request most recent head 86e0239. Consider uploading reports for the commit 86e0239 to get more accurate results

@@            Coverage Diff             @@
##             main     #113      +/-   ##
==========================================
- Coverage   60.04%   59.16%   -0.88%     
==========================================
  Files          39       40       +1     
  Lines        2903     2946      +43     
  Branches      985      995      +10     
==========================================
  Hits         1743     1743              
- Misses        599      642      +43     
  Partials      561      561              

Impacted Files	Coverage Δ
src/dftd4/compat.f90	0.00% <0.00%> (ø)

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