Update mean surf data storage

[sgoodm]

Update how mean surf data is loaded and saved so that it requires minimal options to load existing mean surf for current data/script version and pixel size or save new mean surf for future use

[sgoodm]

Under realistic conditions we will rarely be running this for the same exact subset of data with the same set of options more than once. There are also more options than just country, data version and run version which can affect the mean surface generation. As a result, mean surfaces will need to be generated every single time.

We should keep in the ability to use existing mean surfaces for testing purposes as it significantly reduces run time (and may be useful for some realistic conditions) but the infrastructure should be designed around generating a mean surface every run.