pin tixi3=3.1.1 in windows and mac builds
pin tixi3=3.1.1 in windows and mac builds

To Do:
- Mac does not use the env variable to set the tixi version
- tixi3 remains unpinned for ubuntu build

addresses #709
DLR-SC/tigl,DLR-SC/tigl,DLR-SC/tigl,DLR-SC/tigl,DLR-SC/tigl,DLR-SC/tigl