allow single or multiple grouping variables

[johnmackintosh]

per request from RFL R.U.G

ability to pass one or two grouping variables

This means understanding 1 unquoted variable, and also accepting a character vector and handling that accordingly, ensuring the facets work correctly in both cases

• change grpvar parameter to by - this is keeping with data.table syntax, and should be better understood than 'grpvar'

• update all code to handle joining on one or more grouping variables in addition to the current join conditions

• ensure the correct summary observations for sustained rows where a run supercedes a previously calculated limit

• amend the ggplot2 syntax to handle one unquoted column, or two quoted columns, faceting accordingly