Fixes for Kevin's Init PR

[aliang8]

updates from code review to #167

deleted some comments removed isPolynomialTerm because it exists now in math-nodes

[evykassirer]

you said it was ready, so here are remaining comment to be addressed:

• [ ] removing hasUnsupportedNodes : https://github.com/socraticorg/mathsteps/pull/167/files#r123909501
• [ ] finish conversation about abs check for arg length: https://github.com/socraticorg/mathsteps/pull/167/files#r123319914
• [ ] https://github.com/socraticorg/mathsteps/pull/167/files#r123317967 if you keep hasUnsupportedNodes, can clean up
• [ ] https://github.com/socraticorg/mathsteps/pull/167/files#r123317429 if you keep hasUnsupportedNodes
• [ ] https://github.com/socraticorg/mathsteps/pull/167/files#r123408188 add a isParens to query from math-nodes

I'm not gonna keep going cause I'm pretty much copy pasting every comment :p so if you could look at all the comments in https://github.com/socraticorg/mathsteps/pull/167/files and make sure all the comments have been addressed (you can continue the conversation there) that'd be great thanks!

[kevinbarabash]

@evykassirer @aliang8 I've updated math-nodes to do arity checks on most of the operators, see https://github.com/semantic-math/math-nodes/commit/131d5c36fc1cbc88e699eef060a078d8f130976d. I was thinking of adding checks for mul and add to make sure args.length > 1. Thoughts?

[evykassirer]

@kevinbarabash I like that!