Refactored classmethods, more tests and better custom Nexus support

[xrotwang]

closes #16

[xrotwang]

@lmaurits the fix for #16 is here: https://github.com/lmaurits/phyltr/pull/17/commits/66bc4eb5f6c6d78ff87137e1a3eb9f6d07fe2aea

[codecov-io]

Codecov Report

Merging #17 into develop will increase coverage by 0.62%. The diff coverage is 98.64%.

@@             Coverage Diff             @@
##           develop      #17      +/-   ##
===========================================
+ Coverage     98.8%   99.42%   +0.62%     
===========================================
  Files           28       28              
  Lines         1251     1218      -33     
===========================================
- Hits          1236     1211      -25     
+ Misses          15        7       -8

Impacted Files	Coverage Δ
src/phyltr/commands/stat.py	100% <ø> (ø)	:arrow_up:
src/phyltr/commands/height.py	100% <ø> (ø)	:arrow_up:
src/phyltr/commands/length.py	100% <ø> (ø)	:arrow_up:
src/phyltr/commands/plot.py	96.72% <0%> (+1.56%)	:arrow_up:
src/phyltr/commands/scale.py	100% <100%> (ø)	:arrow_up:
src/phyltr/commands/clades.py	100% <100%> (ø)	:arrow_up:
src/phyltr/commands/base.py	100% <100%> (ø)	:arrow_up:
src/phyltr/commands/dedupe.py	100% <100%> (ø)	:arrow_up:
src/phyltr/commands/cat.py	100% <100%> (ø)	:arrow_up:
src/phyltr/commands/sibling.py	100% <100%> (ø)	:arrow_up:
... and 13 more

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[xrotwang]

I actually increased test coverage by reducing non-covered lines of code :) But since overall line-count came down even more, codecov complains ...