User manual: Spelling, typos

The user manual was spell-checked against the Oxford Dictionary.
Almost all the joined words, e.g. checkbox, dropdown, were
kept, but divelog was changed to dive log. Quite a few tyops
and spelling errors corrected.

A short section on dive plan evaluation was deleted on Robert's
suggestion.

Signed-off-by: Willem Ferguson <willemferguson@zoology.up.ac.za>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>