Tasks cleanup

Replace all tasks that are not part of the main computation to a `threadpool` which can more simply and efficiently schedule small tasks with no dependencies. See #166.

Peter, can you check that this effectively passes all your tests? Thanks!

See merge request !176

Removing duplicate functions to avoid some merge issues for other branches

Master is broken, so back to original for now.

This reverts commit 409531ab.

Removed now-unnecessary large glass file from the repository. All examples download the required glass files from the web-server.

Fix the minimal version of SPH

Ok, I have now fixed the minimalistic implementation of SPH. That's the one that should be used as a template for other people to implement their stuff. It's overly documented and now works on all the test-cases we have in the examples directory.  

Don't know if you want to look at this or not @pdraper given that it does not affect our default setup at all.

Solves #148.

See merge request !218

First really working version of the Minimal SPH implementation. Need more documentation and testing.

First really working version of the Minimal SPH implementation. Need more documentation and testing.

Updated the 3D Sod shock example to use the new glass files. Added script to plot solution. Removed old glass files.

Fixed test125cells when using the Wendland kernels. Neighbour number tolerance was not large enough.

For cells with large counts this matters

Conflicts:
	src/task.c

Need to do this as a pre-init stage to task marking which will unskip tasks that have active cells