Better requirements specification

[ceholden]

Some of the changes associated with #29 and #30 mean that glmnet-python isn't a required package anymore and the broadening of scope for YATSM means that there will eventually be quite a lot of optional dependencies. This project probably needed better documentation for how to install it anyway, but this was the main impetus for the following change:

1. Make requirements/ directory for storing itemized requirements files
2. Put all 100% necessary packages in common.txt
3. Create pointer file requirements.txt that points to requirements/common.txt
4. Add dev.txt for development dependencies (documentation and tests mostly)
5. Group into categories and itemize optional dependencies. Right now this means:
   • accel.txt for making things run faster (Fortran wrapper for enets/lasso, Numba for all purpose fastness)
   • pheno.txt for Eli Melaas' LTM phenology

Finally, add a conda environment file that users can point to for automating the installation. We can probably also use this for travis.ci, which is nice. See this repo, conda-env, for details about the environment.yaml file.