Implement partial FMI 3.0 support

[kyllingstad]

FMI 3.0 has been out for a long time, and FMI Library now has (alpha level) support for it too. We should start working towards supporting it.

I propose a first implementation that covers:

• The level of support we have for FMI 1.0 and FMI 2.0 we have today (which is not complete either!)
• Additional scalar data types in FMI 3.0

Once this is in place, we can work towards supporting other important FMI 3.0 features, in particular array variables. (Separate issues should be created for this.)

[kyllingstad]

I have some funding to work on this this autumn, so I assigned it to myself.

[kyllingstad]

The issue was discussed in today's OSP committee meeting, and the committee approves of the plan outlined in the issue description.

[kyllingstad]

I had to unassign the issue for the time being, as it has been decided that our current project more urgently needs #757.