Feature/interpolate

This pull requests implements the issue #369 with connection to 
 - #251  
 - #252  
 - #254 

 - [x] Lagrange interpolant 
 - [ ] Nedelec interpolant
 - [ ] Raviart Thomas interpolant
 - [ ] Crouzeix-Raviart interpolant (very much like Lagrange)
 - ...