consolidate argument checks for predictSolute

Seems like it ought to be possible to put checks like these:
```r
# Validate arguments
flux.or.conc <- match.arg.loadflex(flux.or.conc)
interval <- match.arg.loadflex(interval)
attach.units <- match.arg.loadflex(attach.units)
lin.or.log <- match.arg.loadflex(lin.or.log)
agg.by <- match.arg(agg.by)
```
in the generic `predictSolute` rather than at the top of every `predictSolute.xx` function. Right? Let's try it. If it doesn't work, make sure `agg.by <- match.arg(agg.by)` gets into predictSolute.loadLm and predictSolute.loadComp and anywhere else it's currently not.