Write index and data file merge program

[tdunning]

It may be desirable to merge many days of data files or grid point index files together to improve query speed.

This should be done by computing intermediate merge files for a week or month at a time. Merging index files should be relatively quick because of their simple structure.

Merging data files should also be fairly quick because all data elements should already be sorted. Having a fixed mapping from grid point to data file might be desirable to facilitate that merging.