BUG: Fix various input parsing bugs related to the distributed jobs

[BvB93]

Most notably:

• Ensure that functional_x and functional_c can actually be None (which is their default).
• Fix the stride and multiplicity keys sneaking their way into distributed auto-generated input files. These keywords are already fully parsed and processed at this point, so they have no business being in the auto-generated input files.
• Expand the test suite such that auto-generated input files (as created by distributed jobs) are validated.

[codecov]

Codecov Report

Merging #393 (5ef2d9a) into master (6751a21) will decrease coverage by 0.23%.
The diff coverage is 84.61%.

@@            Coverage Diff             @@
##           master     #393      +/-   ##
==========================================
- Coverage   83.91%   83.69%   -0.23%     
==========================================
  Files          28       28              
  Lines        2033     2042       +9     
  Branches      305      307       +2     
==========================================
+ Hits         1706     1709       +3     
- Misses        259      266       +7     
+ Partials       68       67       -1     

Files	Coverage Δ
nanoqm/common.py	86.66% <100.00%> (ø)
nanoqm/schedule/components.py	80.88% <100.00%> (ø)
nanoqm/schedule/scheduleCP2K.py	90.00% <100.00%> (ø)
nanoqm/workflows/run_workflow.py	93.93% <100.00%> (ø)
nanoqm/workflows/schemas.py	96.42% <ø> (ø)
nanoqm/workflows/initialization.py	85.60% <80.00%> (+0.10%)	⬆️
nanoqm/workflows/input_validation.py	88.82% <66.66%> (+1.17%)	⬆️
nanoqm/workflows/distribute_jobs.py	89.05% <84.61%> (-5.53%)	⬇️

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