Update install.md

After seeing the confusion that Dr. Evans went through and remembering the same confusion from our co-workers throughout the years, I decided to restructure and rephrase much of install.md. (Not saying this re-write would have fixed or prevented Dr. Evan's issues but I'm sure the poor quality of the doc didn't help) One of the big changes is that I removed the references to pyenv from the document. Pyenv is not included with base Python and venv alone is sufficient for isolating a user's environment from system Python. Plus I feel like it's beyond the scope of our user docs to coach someone on how to properly install and maintain their base Python. I have tested the directions under "Option 2: Venv and pip" to ensure that they provide a useable environment. I have not tested the Anaconda directions since I don't currently have it installed.

A couple of other issues to look at here:

• The "Using geomagio" section is still confusing and written in the original robo-speak, and I don't know if we technically consider geomag.py to be part of the geomagio module. Should we include a couple of quick usage instructions here or have that in another document and provide a link here?
• We should consider removing the "obspy pycurl" portion of the "conda create" command since pip install git+https://code.usgs.gov/ghsc/geomag/geomag-algorithms.git already takes care of any dependencies and it ensures that the correct versions are installed.