Refactor variableAttributeLoader and add new functionality

[fractalsbyx]

Presently variable attributes are stored in several vectors containing index-bool (or more generally index-value) pairs. A separate class 'sortIndexEntryPairList' handles the ordering of the indices using a series of un-templated functions. This can all be done simply by using a std::map instead.

More useful functionality: I would like there to be support for using c++ iterable objects (std::set, vector, list, etc.) for passing equation dependencies rather than one large string. It would also be helpful to be able to add them one-at-a-time and/or by index.

[fractalsbyx]

note-to-self

keep #174 in mind while refactoring