Add user documentation of solve interface

First stab with examples for solving and assembling linear and
non-linear systems along with a little bit on debugging convergence
failures.
Link solving docs into documentation page
Link to solving docs from Helmholtz demo
Add sections on modifying convergence tolerances
Silence sphinx warning
Linkify more things and expand schur complement section
Simplify introduction a little
Add section on default solver parameters
Typo fixes
More typo fixes
PyLit: strip trailing whitespace on blank comment lines
Merge pull request #129 from firedrakeproject/solver-docs
Document the boundary markers on interval meshes.
Fix a bug in the handling of coordinates in 1D vtu files.
Fix spelling.
Beginnings of rst translation of the demo.
Complete burgers documentation
Makefile to produce .py files from all the demos.
Line wrapping
Merge conflicting links in documentation.