install interpolation headers

[[Imported from SVN: r100]]
Added wrapper class P2LocalFiniteElement

[[Imported from SVN: r101]]
Rename method LocalKey::subentity to LocalKey::subEntity.

[[Imported from SVN: r102]]
Switch to new subentity numbering provided by the generic reference elements.

If you change the method calls from subentity to subEntity due to the
renaming in rev102 you should also switch from old to new numbering by
using the corresponding methods in dune-grid. Please refer the transition
docs at http://www.dune-project.org/doc/doxygen/dune-grid-html/group__GridDuneREToGenericRE.html

If you want to keep using the numbering for a while you can use the branch
https://svn.dune-project.org/svn/dune-localfunctions/branches/1.2snapshot

Notice that refinedp1 is covered by pk2d and that the following elements
do not change since their subenities are numbers the same:
edger12d, monom, p0, p11d, p12d, p13d, q12d, q13d, q22d.

[[Imported from SVN: r103]]