Package installation

[smathermather]

We have the beginnings of the requirements for this script in the requirements.txt. But, it'd be great to package this for pip or conda install so it's easier to deploy.

[smathermather]

@hcwinsemius:

Possible command line flags:

• output file name
• path to list of files to merge
• override for null value (currently hard coded to -9999, so also should probably autodetect by default)