Calc module changes: previewing and formulaequationinput

-Change the code within `calc.py` to have a method `parse_algebra` return a parse tree, and have `evaluator` take the tree and evaluate it to a specific number.
-Add a method `latex_preview` to take a tree and give a latex string representing it.

-Create an input type `<formulequationinput>` (modeled after `chemicalequationinput`) that has ajax calls to the preview function above.
-Change studio and other parts of the code base to reference the new input type.