`pollen_data_gen`: support for `subset_paths`

[anshumanmohan]

@susan-garry points out that pollen_data_gen does not currently support subset_paths, which gets in the way of it being a drop-in replacement for the existing ODGI-dependent code. See also, https://github.com/cucapra/pollen/issues/129.

Susan explored the space, saying that we have two options:

• Feed it a subset_paths files along with the GFA graph.
  • Pro: Simpler
  • Con: seems to violate the principle that pollen_data_gen should be precisely a representation of information in a GFA file.
• Leave it as it is, i.e., make it just just a GFA. Then add an extra step to splice in the information about subset_paths.
  • Pro: Keeps the layers separate.
  • Con: More complex.

We have decided to go with the first option.

[sampsyo]

Nice! Sounds perfectly reasonable to me.