Merge pull request #390 from OP2/__init___comes_first_Florian_dammit

__init__ comes first, damn it
Ignore decompression error when reading disk cache

Pickled objects cached on disk are gzip compressed. If this archive
is corrupted (because the write was interrupted or otherwise), the
file exists, but attempting to read it fails with a zlib.error
exception. We simply ignore this failure, which is subsequently
handled as if no value had been found in the cache.
README: clarify quick start instructions
Sketch implementation of "python" par_loops

This allows (for Dats and Globals) par_loops to take a python function
as a kernel argument.  Only very lightly tested.
Copy data back out in python par_loop

When we pass a view into the kernel function, the internal indexed
assignment will create a copy which we then need to splat out to the
actual data.
Raise error for further unimplemented case
Remove overcautious exception
Add tests of python par_loop
Fix python par_loops in conjunction with device backends

We need to explicitly manage moving data from the device and updating
its state.
Merge pull request #392 from OP2/py_parloop

* py_parloop:
  Fix python par_loops in conjunction with device backends
  Add tests of python par_loop
  Remove overcautious exception
  Raise error for further unimplemented case
  Copy data back out in python par_loop
  Sketch implementation of "python" par_loops
make Globals more like Dats

Implement trivial indexing and duplication on Global so it can be used
in places where a Dat is used. This is needed to make the Real
function space work in Firedrake.
ensure the data is up to date
Don't die if a None map is used
Cause the caching to not die in the Global case
get a bit further before crashing
A couple of missing dereferences.
Curse you PETSc4py. Where are your docs!
Refactor the global mat as its own class.
Trick nbytes into working for globals