eigs_cis: input checking

[gfudenberg]

currently the user can pass a phasing_track with any number of columns, but the 4th column is used for phasing. With the default output of bioframe.frac_gc for a cooler bintable, desired GC content would be the 5th column rather than the 4th.

potential solution:

• require that phasing_track has 4 and only 4 columns

[sergpolly]

so i think this is related to #333 - a bit - in CLI this could be addressed by using fname::value_column_name syntax

but ... i guess this isn't about cli ... - maybe we could provide a value column name as a parameter ?
and also regarding frac_gc - isn't the desired value in the 5th column just because typically one gives a bintable to it as an input - i.e. bintabel with the "weight" column ?

[gfudenberg]

I think an idea we discussed was that having a "track format" would simplify API a bit, as users wouldn't need this extra "value_col=track_value_col" argument.