Fixed E being parsed as Eulers in preview

[m-messer]

A slightly hacky fix for E being parsed as E instead of Euler's when using latex2sympy.

Before this PR:

• parsing of typed (text) input was fine, and remains fine.
• parsing of draw/scan (handwriting) input was problematic if it contained 'E'
• The problem was only with the preview, not with the evaluation
• The problem stemmed from the latex2sympy2 library failing to parse 'E' properly
• The latex2sympy2 library is only called when handwriting is used

After this PR:

• typed (text) input behaves as before
• draw/scan (handwriting) input now parses without problems when it contains 'E', and therefore previews succeed and submission is possible
• Evaluation of the submissions behaves as before, with the difference that handwritten inputs with 'E' are now possible to submit.

Issues which resulted in this fix:

1. latex2sympy has E hard-coded as Euler's in its tokens - would have to change the lexer to fix this
2. latex2sympy does this for any value of E in a string, so it cannot use a placeholder such as EEE or Sym_{E}
3. While sympy parse_latex works with E, it simplifies the expression, and results in other tests failing. And does not have any way of disabling this reliably.

I decided that in preview that all instances of E can be parsed as E instead of Euler's in Sympy. As in the preview, we are only checking if it parses, not if it is correct. Since E is passed to the evaluation function as a string, the evaluation side can then treat it as Euler's if required. If preferred, we can do this only when elementary_functions is disabled.

If you can think of any test cases that might break this approach, please let me know, and I will add them.

[peterbjohnson]

This requires a little bit more detail. Maybe at least a link to the PR?

[peterbjohnson]

Worth clarifying in the overall PR description:

• before this PR:
  • parsing of typed (text) input was fine, and remains fine.
  • parsing of draw/scan (handwriting) input was problematic if it contained 'E'
  • the problem was only with the preview, not with the evaluation
  • the problem stemmed from the latex2sympy2 library failing to parse 'E' properly
  • the latex2sympy2 library is only called when handwriting is used
• after this PR:
  • typed (text) input behaves as before
  • draw/scan (handwriting) input now parses without problems when it contains 'E', and therefore previews succeed and submission is possible
  • Evaluation of the submissions behaves as before, with the difference that handwritten inputs with 'E' are now possible to submit.

[m-messer]

Updated description with your comment and linked PR in dev notes.