Fix error when importing hetmech.matrix

[zietzm]

@gwaygenomics made the following observation:

Importing hetmech.matrix without previously importing hetmech.hetmat gives an ImportError

In the following image, I ran the first cell, then restarted the kernel and ran only the second cell. As can be seen below, importing hetmech.matrix alone or before hetmech.hetmat gives this error.

[gwaybio]

I am getting a different, but related error now when using hetmech in a conda environment with:

- pip:
  - git+https://github.com/greenelab/hetmech@b4aca774a2e9b41e6c448535d3b1cc2d9b3f85f4

The error is

---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
<ipython-input-2-96e8d754da50> in <module>
----> 1 import hetmech.hetmat
      2 import hetmech.matrix

ModuleNotFoundError: No module named 'hetmech.hetmat'

It looks like the error is that conda cannot find the hetmat folder:

gway@camo:~/anaconda3/envs/interpret-compression/lib/python3.6/site-packages/hetmech$ ls
degree_group.py  degree_weight.py  diffusion.py  __init__.py  matrix.py  pipeline.py  __pycache__  xarray.py