Add symmetric gradient operators

[pjaap]

This is my current idea how to add a high level symmetric gradient operator.

As we discussed, I want to highlight the Voigt notation for the user in the operator names.

Now, we have

• symgrad_voigt and εV with u und factor as arguments.
• no default for the parameter factor, since there is no canonical Voigt transformation (stress vs. strain: 1.0 vs. 0.5)

It plays well in my project, so I think it may be ready for merging.

[chmerdon]

Looks good, feel free to merge when Tests are finished.