Minor fixes

[Anaphory]

This pull request suggests three minor fixes:

• Do not catch exceptions when Beastling is run from python -m, in order to make debugging easier
• Move some binary model properties up in the type hierarchy, without actually changing much
• Fix one minor issue in the documentation

If I find the opportunity to add tests or other documentation fixes (eg. #202 #209)

This is a duplicate of #210, except now I picked the correct base branch here.

[codecov-io]

Codecov Report

Merging #211 into develop will increase coverage by 0.76%. The diff coverage is 94.11%.

@@             Coverage Diff             @@
##           develop     #211      +/-   ##
===========================================
+ Coverage     91.9%   92.66%   +0.76%     
===========================================
  Files           21       21              
  Lines         2605     2577      -28     
===========================================
- Hits          2394     2388       -6     
+ Misses         211      189      -22

Impacted Files	Coverage Δ
beastling/__main__.py	0% <0%> (ø)	:arrow_up:
beastling/models/covarion.py	98.76% <100%> (+12.31%)	:arrow_up:
beastling/models/binary.py	90.65% <100%> (+5.8%)	:arrow_up:
beastling/cli.py	90.36% <100%> (+0.11%)	:arrow_up:
beastling/configuration.py	92% <0%> (+0.14%)	:arrow_up:
beastling/beastxml.py	88.52% <0%> (+0.44%)	:arrow_up:
beastling/models/basemodel.py	95.1% <0%> (+0.81%)	:arrow_up:

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[Anaphory]

I don't know how I would reasonably test the __main__ error handling procedure. Any suggenstions, @lmaurits and @xrotwang, or may I merge this as-is?

[xrotwang]

AFAICT, the only course of action taken by BEASTling upon encountering the new exception class, is terminating execution. So I guess, no tests are required, there either will be regular script termination or an error will be thrown - and the script will terminate.

[Anaphory]

The idea is that this new Exception subclass is different from existing subclasses, so errors in

try:
   ...
except wrap_errors as e:
    errmsg("Error encountered while parsing configuration file:\n")
    traceback.print_exc()
    sys.exit(2)

are only caught when cli.wrap_errors has its default value Exception, not when __main__ came along and redefined cli.wrap_errors, but the result (termination, which is the desired effect) is exactly as you describe. (NoneException will not be raised.)