Remove all unnecessary code from aggregateSolute

[aappling-usgs]

Issue #199 and PR #219 are converting aggregateSolute to an internal function (deprecated for external use) and avoiding using it for uncertainty calculations. This is avoidance is appropriate even if someone doggedly keeps calling it as loadflex:::aggregateSolute, so I think we can take out a ton of the code currently in this function. A minimal function is a maintainable function. Arguments that are no longer necessary can be removed, leaving in a ... argument, then checking ... for named arguments that have been removed and giving an informative warning about them.