Add assembleGradient() method to FE assemblers

This adds a method to compute only the gradient.

On the pro side it saves a lot of time if only the gradient is needed (for first order methods).

On the con side it introduces some duplicated code (but only for hyberdual numbers).

A test case is added.

assigned to @patrick.jaap_at_tu-dresden.de

added 4 commits

• 56b737f3...ee54179d - 2 commits from branch agnumpde:master
• 6b9bdb9a - whitespace cleanup
• e623a760 - Add assembleGradient() method to FE assemblers

Compare with previous version

added 5 commits

• e623a760...e5407289 - 3 commits from branch agnumpde:master
• 950f4a03 - whitespace cleanup
• 32b93767 - Add assembleGradient() method to FE assemblers

Compare with previous version

mentioned in commit 8955c6d5

merged