Need better math replacements for English texts

[matze-dd]

The replacement collection in variable parms.display_math works
quite well, if German is the main language. Requirements for replacements are summarized in the script in function set_language_de().
Till now, we could not yet select replacements that work equally well
with the English version of LanguageTool. For example, sensitivity is not good with the collection provided in function set_language_en() in these cases:

• missing final dot in an equation, if something like 'Therefore'
  is following;
• lower-case text continuation after an equation with final dot.

[matze-dd]

See #22.