utils.trim: Trim a docstring according to PEP 257
mpi.collective decorator deals correctly with missing docstrings
Move PETSc MatMPIAIJSetPreallocation reference to avoid sphinx error
Add DataSet and subsections to user documentation
Remove redundant function ParLoop.generated_code
Make Set.{CORE,OWNED,IMPORT_EXEC,IMPORT_NON_EXEC}_SIZE private
Minor doc fixes in base
Some more docstrings for the device module
Add a brief description to the pyop2 package
Merge pull request #215 from OP2/doc-fixes

Minor doc fixes, remove redundant function ParLoop.generated_code