Check whether optional new features can be added to 3.0 as a minor release

[pmai]

From the Design Meeting on 2022-01-19: Go through planned optional features, like fast pointer-based variable access #515, or enhanced step-size agreement for intermediate update, and check whether the current wording in the FMI 3.0 standard would properly allow adding them as optional features in a 3.1 release.

[pmai]

As discussed in the plugfest session on 2022-01-25, this can be done for all of the currently envisioned features with the following clarifcations/enhancements:

• Allow additional entry points in FMU code, specify restrictions #1643
• Allow additional capability flags, additional elements in XSD #1644

The only restrictions that we see is that supporting e.g. sparse arrays (#1098) it might be that one would want/need non-backward compatible changes to things like start values that are already in 3.0; those would need to be performed in a major release.