site<- and horizons<- optional specification of explicit join keys

[brownag]

site<- and horizons<- SPC methods have always joined on all matching column names. (i.e. merge is called with no by argument)

It would be good to optionally support a by, or a by.x and by.y, argument on the left hand side to these to allow for differences in column name between existing slot data and incoming data, as well as to allow folks to be very explicit about join conditions where there might be unintended conflicts or similar

[brownag]

We could also consider being a bit more verbose like the dplyr::*_join functions and put out a message when the join columns are inferred (that would require some more bookkeeping than we currently have)