Remove all unnecessary code from aggregateSolute

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.