Add support for generic column data

[brittonsmith]

PR Summary

This PR is in response to Issue #159. This adds support for loading merger trees from generic ascii files that minimally define a unique id and descendent id for each halo. Data needs to have the following attributes to be loaded:

• data in a single ascii file ending in .col with a row for each halo
• a three line header (each line starting with "#") defining field names, data types, and units.

For example, something like the following:

# uid, desc_uid, mass, redshift
# int, int, float, float
# None, None, Msun, None
1457223360,-1,1.023166e+12,0.000000e+00
1452164856,1457223360,3.164029e+11,1.019287e-02
1447024182,1452164856,1.913669e+11,2.050185e-02
1441813042,1447024182,1.929496e+11,3.090656e-02

@PauCaBu, please give this a try and let me know what you think. I would be happy to iterate on this with you.

PR Checklist

• Code passes tests.
• New features are documented with docstrings and narrative docs.
• Tests added for fixed bugs or new features.