load datamodels once and only once and only when/if needed

[dbarrosop]

Before we would:

1. Load a datamodel per driver
2. Load all datamodels when loading ntc-rosetta

this lead to (a) wasting memory (b) wasting initialization time.

Now we:

1. Load each datamodel once and only once, regardless of how many drivers use the same datamodel.
2. Load datamodels when/if needed