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
implement NaryFunction
add substr(str, begin, end) function.
add `len(str)` function.
add head and tail function.
use LESSER and GREATER to simplify the head and tail function
clean test query.
didn't propagate hashjoin indexing fix to all implementations
failing test
Merge branch 'bmyerz-complete-indexing-fix' into bmyerz-join-index-test
rest of test
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