new 'experimental.md' doc for proposed updates to dataset.md

[smrgeoinfo]

Based on Discussions at SDO cluster meeting Monday, created new doc, move advanced topics to experimental, add stuff from variableMeasured Tier 4 and anvanced topics from Issue 27 branch. This is to clear the way for the issue27 branch to merge with dev (#168)
addresses #189

[ashepherd]

Thank you Steve! Do you think we should link to it from the Dataset guideline somehow?

[ashepherd]

just changed the base of this PR to be develop

[ashepherd]

• Add description of what this Experimental document is
• Add Table of Contents (by class Dataset, DataRepository, etc. ten by each applicable section)
• Link to this from top-level README

[smrgeoinfo]

@ashepherd suggestions implemented in Issue189-ExperimentalRecs branch

[datadavev]

A couple minor edits needed in the examples in Experimental.md as per comment

[mbjones]

From 2022-02-07 SOSO meeting:

• Steve made updates following the recs last meetings
• TODO: Examples need to follow the new context loading guidelines
• TODO: the IGSN @id example should use the “igsn:” prefix
• Needs review and merge

[mbjones]

@smrgeoinfo Looks good. A few suggested changes, but particularly the context update (which I think I have right in the propose change but was hard to check the syntax in the issue editor).

[mbjones]

LGTM.