Fix mhpl8r diff command

[standage]

The profile.alleles() function returns None when a profile has no alleles for a marker. The diff module applies set operations on the output of this function without any error checking, which is problematic when a None is returned. This update modifies the function to return an empty set instead of None, so that set operations will work as expected.

The mhpl8r diff command was also updated so that each allele is printed on a dedicated line, rather than joined on the same line.

This update incidentally includes a few small changes to the location of certain package files.

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• [x] Changes are clearly described above
• [x] Any relevant issue threads are referenced in the description
• [x] Any new features are tested (see docs/DEVEL.md for details)
• [ ] Substantial changes are documented in CHANGELOG.md (see https://keepachangelog.com/en/1.0.0/)

[codecov-io]

Codecov Report

Merging #75 into master will increase coverage by 1.81%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #75      +/-   ##
==========================================
+ Coverage   95.76%   97.57%   +1.81%     
==========================================
  Files          25       25              
  Lines         803      743      -60     
  Branches      158      141      -17     
==========================================
- Hits          769      725      -44     
+ Misses         21        8      -13     
+ Partials       13       10       -3

Impacted Files	Coverage Δ
microhapulator/panel.py	91.66% <ø> (+9.46%)	:arrow_up:
microhapulator/sim.py	100% <100%> (ø)	:arrow_up:
microhapulator/__init__.py	93.02% <100%> (-0.32%)	:arrow_down:

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