Add stub for a clock with a prior on rates

[Anaphory]

This does not need to be merged yet, it's still very much WIP and only here to inform you about its happening!

[codecov-io]

Codecov Report

Merging #191 into develop will increase coverage by 0.22%. The diff coverage is 92%.

@@             Coverage Diff             @@
##           develop     #191      +/-   ##
===========================================
+ Coverage    91.85%   92.08%   +0.22%     
===========================================
  Files           19       21       +2     
  Lines         2518     2565      +47     
===========================================
+ Hits          2313     2362      +49     
+ Misses         205      203       -2

Impacted Files	Coverage Δ
beastling/beast_maps.py	100% <100%> (ø)	:arrow_up:
beastling/report.py	94.5% <100%> (ø)	:arrow_up:
beastling/beastxml.py	95.36% <100%> (+0.29%)	:arrow_up:
beastling/distributions.py	90.58% <90.58%> (ø)
beastling/configuration.py	89.61% <92.85%> (+0.63%)	:arrow_up:
beastling/clocks/prior.py	95% <95%> (ø)

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[Anaphory]

This is now nearly complete, the missing bit is the equivalent of super().__init__ for python2.

[Anaphory]

Done!

Fixes #192 for the strict clock. Does first steps to address #119 by providing a framework for distributions.

[lmaurits]

You've been busy!