Add min (least) and max (greatest) expressions
var-ary greatest and least functions,add general support for var-ary functions
PEP8
add greater and lesser functions
Move check to a sensible place
Make lookups in expressions library case insensitive. Fixes #166
Typo
Merge branch 'master' into domoritz-min-max
expressions_library: refactor with reduce

See https://github.com/uwescience/datalogcompiler/pull/164#commitcomment-5986637
Merge pull request #164 from uwescience/domoritz-min-max

Add min (least) and max (greatest) expressions