Revert "Override default petsc4py error handler"

This reverts commit c0107bc5cc0814b26ed31d3bf3e04e742c66b54b.

The mpiabort error handler turns out to be more trouble than it's worth,
since it hides any useful stack traces we might see.  Having found the
cause of the bug that this change was originally made to catch, we can
safely revert it.
wrap finite element creation to support quadrilaterals
make _dofs_per_entity calculation quad-compatible
cell_node_list for quadrilaterals
refactor entity_dofs flattening
quadrilateral utility mesh creation
fix boundary domains for quadrilateral meshes
fix output for quadrilateral meshes
solve edge orientation problem
test cases for quadrilateral meshes
FunctionSpace-independent _cell_closure calculation
introduce QuadrilateralMesh class
refactor cell_node_list creation

The affected code snippet is a canonical code smell (Switch Statements
[1]). Further details, for example at [2].

[1] http://c2.com/cgi/wiki?SwitchStatementsSmell
[2] http://sourcemaking.com/refactoring/replace-conditional-with-polymorphism
refactor cell_closure
refactor facet_dimension for boundary treatment
add docstrings
refactor cell type detection

- Eradicate cellname optional keyword argument
- Redefine fiat_utils._cells to use topological dimension instead of
  geometric dimension
fix misleading variable name

That is not the number of vertices per cell,
that is the number of facets per cell.
test case for reading quad mesh from file

Failing test case at the moment.
fix reading quadrilateral mesh from file

Let's see if it passes code review.