Big update to custom expression feature.

 - supports range and index subexpressions and many new operators
 - switched to my patched version of exp4j to support all this.
 - expressions belong to rocket document. Accessed from analysis menu.
 - expression importing from file
 - datatypes section defined in file for storing datatypes other than internal ones
 - flightdatatype fix to forget outdated types
 - many GUI fixes to custom expressions
 - new unitgroups supported. Auto unit detection for SI units in custom expressions.

Had to carefully merge loading/saving code with Kevins recent de-localization update. Hopefully changes to materials saving kept but switched datatype access to just using symbol as the key.

Hopefully can get the changes to exp4j upstream so we don't need to keep using this patched version.

git-svn-id: https://openrocket.svn.sourceforge.net/svnroot/openrocket/trunk@953 180e2498-e6e9-4542-8430-84ac67f01cd8