Added support for auto-creation of conda environments

[jraviotta]

This is one possible resolution for #168 .

This PR updates the Makefile requirements target with logic for selecting package and environment managers. If conda is detected, a conda environment is created and then updated with a new environment.yml file. If conda is not detected, requirements.txt is installed with pip and then an environment is created with virtualenv as before.

The conda condition successfully creates the environment and the local package src is available in the jupyter kernel in jupyter lab. I can run the notebook cell described in #168 without an error.

I have not tested the virtualenv condition as I am unfamiliar with it's normal behavior.

[pjbull]

Thanks for the interest in this feature @jraviotta!

We're not going to merge this as-is, since we're moving to allowing the user to pick an option rather than bundling everything together. (see #162) If you want to test the changes on that branch with your conda flow, it would be great to hear the results.