Make FiniteElement::local_coordinate_of_node(...) const

INTERFACE CHANGE: If it is overloaded anywhere in your personal code it
will need to be made const there as well.
Add interface for Integral::knot(..) returning vector

* So much easier to work directly with vectors instead of looping over
  entries and copying.

* Not inefficient because of return value optimisation (returned vectors
  are not copied unnecessarily).