Remove Merged Branches

[lcjohnso]

It is difficult to track on-going development (especially concerning documentation) due to the large number of branches that have previously been merged but not removed from the codebase.

Here is a list of branches that appear ready for pruning:
py3_compat merged by #113
adjust_dust_prior by #104
doc_update by #99
travis_ci_setup by #96
readthedocs_fixes by #92, #91, #90, #89, #88
doc_fileformats by #87
dtu_plots by #86

[karllark]

This is a good point. I've been on the fence on if I should prune or not. This pushes me over. They always can be recovered of course.

[karllark]

Done. Very useful as the list of branches now only shows active ones. And I can see I have two more active branches than I thought. Good reminder to do something with those branches.