Add the eltype of time series data to the metadata structs

[daniel-thom]

We are currently serializing string versions of time series data shapes to HDF5 attributes. This is causing problems with the new PRs from @GabrielKS. We can instead embed these eltypes into the metadata structs and handle their serialization/de-serialization in the normal way.

[jd-lara]

Deferred to later in IS 3.0