Release MGET as a conda package on conda-forge

[jjrob]

ArcGIS Pro uses Anaconda Python and conda for package management. To support this community, we should release MGET as a conda package on conda-forge. Doing so will simplify the MGET installation experience for ArcGIS Pro users, and should even allow it to be installed from the ArcGIS Package Manager GUI.

[jjrob]

Done and tested.

• https://anaconda.org/conda-forge/mget3
• https://github.com/conda-forge/mget3-feedstock

We still need to update the installation instructions, but we'll handle this as a separate issue (#14).