use relative imports inside MDPOW

[orbeckst]

• fix #245
• avoid circular imports involving the workflows.registry

[orbeckst]

@cadeduckworth have a look at this PR. These relative imports work, at least in local testing. If this looks good to you, approve and merge.

[codecov[bot]]

Codecov Report

Merging #257 (7ce55c4) into develop (afb1ced) will decrease coverage by 0.02%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #257      +/-   ##
===========================================
- Coverage    80.74%   80.73%   -0.02%     
===========================================
  Files           15       15              
  Lines         1906     1905       -1     
  Branches       294      294              
===========================================
- Hits          1539     1538       -1     
  Misses         276      276              
  Partials        91       91              

Impacted Files	Coverage Δ
mdpow/workflows/base.py	81.63% <100.00%> (ø)
mdpow/workflows/dihedrals.py	94.87% <100.00%> (-0.05%)	:arrow_down:
mdpow/workflows/registry.py	100.00% <100.00%> (ø)

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