bedMaxWig dealing with mulit wig struct per chrom

[cshoben]

Edit chromSlice to deal with multiple wigs per chrom by being a place to hold all the data for a single chrom in one place (easiest fix)
Alternatively, sorting both files and reading them simultaneously would be the fanciest way to do this. (recommendation by Craig)

See the wig update mem usage pull request from early June 2021 for the full conversation.