Calc module changes: previewing and <formulaequationinput>

How to see it
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Make a "Numerical Input" or "Advanced > Math Expression Input" problem in studio. By default, it should use the new input type.

Alteratively add a `<formulaequationinput/>` to a `<numericalresponse>` or `<formularesponse>` in some problem XML.

Also, if you have the `content-demos` course in your setup, checkout the `peterb/formula-eq-input` branch, and navigate to the 'Formula Equation Input' sequential.

Changes (in brief)
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* Change the way `evaluator` in `calc.py` works.
* Add a method `latex_preview` to take a tree and give a latex string representing it.
* Create an input type `formulaequationinput` with associated javascript that calls `latex_preview` on user input.
* Change studio and other parts of the code base to reference the new input type.

Reviewers
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* @sarina 
* @nedbat 
* Please find someone from Studio team to review Studio changes
* @auraz or someone from Kiev team, as well.