Add deprecation messaging for aggregateSolute

[aappling-usgs]

Add info explaining to users that aggregateSolute is deprecated (for external use). Places this info can go:

1. Add aggregateSolute to the list at ?loadflex-deprecated. The current example to work from is exampleMetadata, with relevant roxygen lines here: https://github.com/USGS-R/loadflex/blob/master/R/examples.R#L60-L62. The new roxygen2 block for this redirection should probably describe NULL rather than the current function, which will allow us to keep the current documentation available for internal use.

2. Move the internal documentation to some other @rdname (or is it @name?) so that if someone types ?aggregateSolute they get directed straight to the deprecation help page. Rename the internal doc to something similar (aggregateSolute-internal, maybe?) so that we devs can still check it out when needed.

3. Add a deprecation check and warning to the aggregateSolute function. We don't want to throw the warning when aggregateSolute is called internaly (e.g., by predictSolute), so probably model this check/warning on https://github.com/USGS-R/streamMetabolizer/blob/35407ac81fd7777c3664f64ba27ced31f0b63e7e/R/convert_date_to_doyhr.R#L17-L23.