FMRC on-disk cache

[lesserwhirls]

In 5.x we replaced the on-disk cache from GridDatasetInv for FMRCs in favor of an in-memory guava cache. The TDS really needs to be able to persist the GridDatasetInv cache between reboots, so I'm going to add it back...sort of. Rather than add an implementation to netCDF-Java, I'm thinking of adding a small service provider hook in GridDatasetInv, specifically down in GenerateInv, to feed the in-memory cache from an on-disk cache, if an on-disk cache implementation is found. In this way, we can enable that functionally for the TDS without adding a dependency (one of the reasons we removed it to begin with) The service provider would take an MFile or String (path associated with MFile) and return a GridDatasetInv object.