Fix/e latex preview (#236)

* Fixed E being parsed as Eulers in preview

* Removed E from the list of potentially placeholders

* Added note to dev.md for `E` parsing and added more tests for Euler preview

* Added link to PR

* Swtiched test to relative import

Author: m-messer

app/evaluation_tests.py

@@ -1,8 +1,8 @@
 import pytest
 import os
 
-from app.preview import preview_function
-from app.utility.preview_utilities import Params
+from .preview import preview_function
+from .utility.preview_utilities import Params
 from .evaluation import evaluation_function