Retain data loaded from disk for multiple Calcs in a CalcSuite, rather than reloading each time

[spencerahill]

• Determines what the needed arrays and time subsets are for each computation, and then holds on to a copy whenever a subsequent Calc needs one identical to (or a subset of) a pre-loaded one
• So would keep the loading functions within the Calc class, but would have the Calc check to see if the CalcSuite has the data first. OR that check is done within CalcSuite -- if it doesn't have the data already, it calls on the Calc's loading functions to get the data.

Currently, the netCDF data is loaded and freed with each Calc, even if all of them require the same data, which slows things down a lot, especially for larger computations.

[spencerahill]

A nascent form of this functionality has been implemented in the aospy-obj-lib repo, but it is buggy and messy.

Also, important that the location-specific file access methods (e.g. the dmget or hsmget calls required to free files saved on the /archive of NOAA GFDL) be factored out of the other code to the fullest extent possible. Eventually want to support users at other institutions, and enable them to make their necessary system calls as they deem necessary.

[spencerahill]

This is partially being implemented in #155.

However, the aspect of holding onto data across calculations, which would definitely be useful, is not addressed there. So leaving this open.