[MaL] Zero and nan expressions for Kelvin vec/mat

Add KVnan, KVzero, KMnan, and KMzero to MathLib::KelvinVector namespace. Often used for initialization.

Also extract nan alias from individual processes into ProcessLib::ConstitutiveRelations.

Closes #3457 (closed)

1. Feature description was added to the changelog
2. Tests covering your feature were added? Existing tests.
3. Any new feature or behaviour change was documented?