Started checksum problem file
Added input and output stacks, as well as some relevant instructions
Updated odd problem for input stack
Number-IO updated for input/output stack
Merge branch 'io-stacks' into io-stacks-prog-synth
Changed "demos" problem files to use new input stack.
Updated regression problems for input stack
Merge branch 'io-stacks' into behavioral-diversity

Conflicts:
	src/clojush/problems/integer_regression/factorial.clj
Merge branch 'behavioral-diversity' into prog-synth-benchmarks
Added registered-for-stacks, which returns all instructions for a list of stacks
Merge branch 'behavioral-diversity' into prog-synth-benchmarks
Moved Levenshtein distance to util, and added median problem file
Changed problem atom-generators for number-IO and median to use new fn
Added 'use of clojure.set
Merge branch 'behavioral-diversity' into prog-synth-benchmarks
Finished median problem and fixed small typos in number-IO.clj