Extend the CSM ISD Specification

[jlaura]

Two comments that recently came in on a very helpful paper review that are valuable to consider and would require an extension to the CSM ISD spec.

1. The ISD does not currently allow user defined uncertainty information inside of the ISD. The reviewer rightly pointed out that this is limiting. How might we add uncertainty information?
2. I know that this is something we have tossed about a bit and I would like to resurrect. The reviewer notes that the image identifier is not included in the ISD. This is because the implicitly link the ISD to the image (same directory, same base name, different extension). At a minimum, we should consider adding the image identifier to the ISD explicitly, removing the implicit mapping.

[jessemapel]

@jlaura This should probably go on usgscsm