PH5 last update date field

[nick-falco]

Is your feature request related to a problem? Please describe.
Currently there is no way to tell when a PH5 experiment was last modified.

Describe the solution you'd like
Add a "last updated" field to the experiment table. This should be changed whenever metadata or timeseries data is modified. Additionally we will want a tool to update existing experiments to include the new field.

[timronan]

This is a good idea that should be considered in the GeoHDF project, but this enhancement should probably not be made in PH5 unless it is necessary for the migration.