standardize_variables: Use lookup table of parameters rather than text-matching

[stephhazlitt]

Need to construct a table matching parameters in rems::ems_paramaters to those in wqbc::limits

limit_name	ems_parameter	ems_param_code
X	X Total	123
X	X Dissolved	1234

Then standardize_variables should lookup the limits in wqbc::limits using this table, then output from calc_limits should look like this:

Variable	Value	LimitName	UpperLimit	LowerLimit
X Total	7	X	11	2
X Dissolved	6	X	11	2
X Suspended	5	X	11	2
X Foo	42	X	NA	NA

X Foo was not matched in the lookup table

Then calc_limits should have an argument drop saying whether or not to drop the rows where limits were not able to be calculated - either because it wasn't able to be matched in the lookup table, or the limit was dependent on another variable that was not available.