ts: Add/use seqLength to properly work with tree sequences

[gregorgorjanc]

Much/all of code with tree sequences relies on having sequence_length. Such a property is not part of AlphaSimR, but we will need it, so this issue is to remind ourselves about it. Maybe it's as simple as bringing ts object into SimParam class and then we will have:

ts = ts_load(ts_file) # using RcppTskit::ts_load()
founderGenomes = asMapPop(ts, ...)
SP = SimParam$new(founderGenomes, ts)
SP$ts$sequence_length() # using RcppTskit::TreeSequence$sequence_length()

@hannesbecher @LynxJinyangii is there anything else we would actually have to do in addition to having access to SP$ts$sequence_length()?