add MixingVolumes

[Mathadon]

This issue is to add the MixingVolume model to the library.

[Mathadon]

Looking into this. The merge script has a bug when adding new files since the git-diff does not recognize the not-yet-copied file.

[dhblum]

Is the bug addressed currently in #10? Or you are still looking into it?

[Mathadon]

The bug has been fixed and pushed to https://github.com/Mathadon/BuildingsPy/tree/issue278_mergeMpc.