update MCF record lifecycle dates

[tomkralidis]

This PR updates pygeometa's record lifecycle support by moving from metadata.datestamp to metadata.dates, where metadata.dates is the exact same design pattern as identification.dates, but focused on record lifecycle (vs. identification.dates which is based on citation).

Note that this is a breaking change, and that the 0.20.0 release can be used safely until users migrate accordingly against master or when 0.21.0 is released. Summary of changes are as follows:

• metadata.datestamp is removed (BREAKING)
• identification.dates is now optional (BREAKING)
• metadata.dates.creation is added and mandatory (BREAKING)
• metadata.dates.revision is added and optional

[pvgenuchten]

this indeed is quite impactfull on existing mcf's

for future changes may be interesting to support a migration automation

if 'datestamp' in mcf['metadata']:
   mcf['metadata']['dates'] = {'revision': mcf['metadata'].pop('datestamp')}