Main bulk of stuff needed to update strain rate in momentum diagnostics. I've tested that the code compiles and runs, but need to tidy the code and verify that it also work in sync with the viscosity, as planned.
Minor comment clarification.
Some tidying up following my chat to Dr. Wilson. Need to test this next in a non-linear run, to verify that things are being updates as I think they are.
Merge trunk into branch.
Merge in trunk.
Fixing bug #1262890: Individual lagrangian detectors are initialised with wrong names, causing detector I/O to fail in parallel.
Fixing re-initialisation of detector names from checkpoint.
Adressing bug 1260019 through improved detector buffer handling

Use separate send and receive buffers for each target proc 
in exchange_detectors() to avoid buffer corruption.
Fixes for fluidity with petsc 3.4:
  
* MatRestoreRow() now zeros ncols upon return so we have to make sure we put the call after the last use of ncols
* the error handler in the test_petsc_csr_matrix unittest is moved inside petsc_tools - so we can use a module 
  variable to test whether it has been called or not.
Commit purely in tests: On Rhodri's advice, modifying test criteria to accept
both PETSc 3.1 and PETSc 3.4 solver iteration values.
Change in tests only: typo in previous commit set a ridiculous test value.
Commit in test only: Minor tolerance relaxation.
Change only in test: On Cian's advice, until the gmsh team respond with a more
useful fix, altering the geo file slightly to get the test to pass here. This
should be reverted when gmsh is fixed.
Reverting 4310 as it had the unintended effect of breaking trunk on precise.
Correcting the fix for the gmsh failures on trusty.  Temporary: should be reverted if/when gmsh responds.
Speedup vtudiff by only setting up the probe once (instead of once per field).
Remove "field manipulation" methods from vtu class.

These are methods are all vectorial operations like SubFieldFromField that basically perform fieldA-fieldB in a very inefficient way. You should simply use numpy vector operations here.
relaxing some test tolerances to pass since the upgrade to Petsc 3.4.
Merge from trunk
Modify this test suc htaht results are consistent with and without debugging enabled (and also such that it passes on red hat). Thanks for Stephan and Tim for input.