[numerics] Remove some maths typesetting idiosyncrasies.

* Remove several instances of manual spacing adjustments,
  where LaTeX's default spacing works just fine and is more
  consistent.
* Replace \mbox'es with more idiomatic \text.
* Improve grammar by adding commas before "where".
* Improve source code by removing pointless newline-escapes
  and minor rearrangements.

Author: tkoeppe