desiconda used for 20200801 and 20.7 software release

[sbailey]

This PR documents the changes used for the desiconda 20200801-1.4.0-spec installation used as the base for the DESI 20.7 software release.

The primary goals were to

• upgrade to python 3.8.x, fitsio 1.x, astropy 4.x, bokeh 2.x
• add iminuit (for future desitarget) and pycuda (for perlmutter prep)
• add for convenience / experimentation: papermill, dask/distributed, sphinx

This environment is available at NERSC via

source /global/common/software/desi/cori/desiconda/20200801-1.4.0-spec/setup_desiconda.sh

Or including all the desi packages

source /global/cfs/cdirs/desi/software/desi_environment.sh 20.7