Add PMC data filter functions

lts(metric), sts(metric), sb(metric) all return the PMC
value for the metric in question for the date of the ride
and therefore allow you to select rides based upon PMC data.

e.g. sb(TSS) > -5

could be considered a way of looking for rides when you were
in a well tapered state, or at least, when you should be able
to perform well.

or, e.g. lts(TSS) > 100

could be a way of looking for rides at peak form (if 100 is
a peak CTL for you).