Merge the devel branch.
Finish merge with devel.
Add the precalculated_forms.
Add description.
Add missing files.
Small updates of Selector, Spline.
Changes in Spline:

 - constructor does not call calculate_coeffs
 - calculate_coeffs does not return true / false
Space::isOkay() also checks if the instance is up-to-date.
Add basic element-wise parameters setup.
Fix in constant forms.
Add test example 12.
Fix in new test example.
Fix caching for OpenMP assembling.
Fix of Boundary Conditions.
Progress towards element-wise constant parameters for nonlinear forms.
ElementWise parameters finish the design.
A fix.
Decrease the integration order for derivatives.
Fix a memory leask.
Add Element::get_center().