[WIP] Visualization of quadrature functions

[najlkin]

This PR adds support for visualization of quadratures (QuadratureFunction). They can be loaded from a file through the new command line argument -q or in a socket stream with the keyword quadrature (instead of solution). Two options of visualization are offered (which can be switched by Q key):

• Piece-wise constant function on a refined mesh (closed Gauss-Legendre refinement)
• Discontinuous elements with DOFs collocated with the quadrature on the original mesh.

TODO 📑 :

• [x] a key for switching
• [x] maintain consistency of quadrature / grid function
• [ ] testing
  • [x] 1D
  • [x] vector
  • [x] stream
  • [ ] scripts
  • [ ] parallel
• [x] proper refinement (ClosedGL?)
• [x] check for tensor basis
• [x] README, CHANGELOG