Factor base into a separate repo

[kkappler]

mt_metadata is a quite useful framework that could be extended to many other metadata applications, both geophysical and in other branches of data science and analysis.

It may be worthwhile factoring the base class and a few example classes in time series into a separate repository so that mt_metadata is a specific application of the more general approach.

[kkappler]

Or, a future version could be based on pydantic