Allow users to specify samples to include (and ancestral sample)

[dwinter]

At present we use all samples in a BAM file, but there may well be cases in which it make sense to exclude some samples (actually the Tt data is a case in point!)

We also assume that the ancestor is the first sample off the ReadDataVector, which is not always going to be the case

• [x] Allow user to specify name of samples to include via config file(and possible warn if some samples in the read-group dictionary are being skipped)
• [x] Make the ancestral sample a special case
• [ ] Possibly modify the site data struct to include ancestral reads as a separate element

(this is related to #6 since the data-collecting function would have to accomidate this change)

[dwinter]

Will require ReadData collecting function (#6) to be able to skip reads that aren't needed (not something it can do at the moment)

[dwinter]

Rather than changing theModelnput struct I'm explicitedly putting the specified-ancestral reads in the 0th positions of the ReadDataVector.