Specify positions for branch LD matrix

Introduces the ability to subset the branch LD matrix with genomic
positions. This is a little trickier than subsetting the LD matrix with
sites because positions can map to duplicate tree indices. We still only
allow unique, sorted positions as input. For instance, if we receive
`[0.1, 0.2, 0.3, 2.5, 2.8]` from a user, it may map to `[0, 0, 0, 2, 2]`.
To this end, I've repurposed the `check_sites` validation code and added
some logic for dealing with index repeats.

For now, until I devise some simple optimizations for matrix filling,
we're filling a square matrix entry by entry. If we are skipping trees
with our specified positions, we must iterate over the entire range
regardless because we're keeping a running sum of the statistic and we
need the intermediate state regardless of whether or not we're keeping
it.