Small kmath enhancements for movablepoint1

Summary:
added k*.is to test whether something is a valid operand to k*
added rounding to knumber/kvector/kpoint
(used for movablepoint2 snapping/bounding)

Test Plan: Seems to work in movablepoint2

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6853