Issue 633 add klu

[Scottmar93]

Description

Added pybind11 interface to sundials with the sparse linear solver KLU works for all models including 2p1D now. Still fairly simple requiring the jacobian and events to be passed in but we can add these as options later. Additional things added:

• a unit test to ensure it works on one of the standard examples included in sundials (ida_roberts_klu)
• an integration test to ensure it works on the spme (in 1D). (I didn't add the 2p1D case because that takes too long)
• an install script in "scripts/install_sundials_4.1.0.sh" that downloads sparsesuite, sundials4.1.0, sets up pybind11, and then generates the klu solver shared library so that it can be imported in python
• some simple instructions in the README.md for how to install using the install script
• added KLU to travis.yml (I think I have done this right but please check) (Unfortunately this will add quite a bit of time to tests because needs to download and install SuiteSparse which takes a while).

Fixes #633

Type of Change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

Key checklist:

• No style issues: $ flake8
• All tests pass: $ python run-tests.py --unit
• The documentation builds: $ cd docs and then $ make clean; make html

You can run all three at once, using $ python run-tests.py --quick.

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works
• Any dependent changes have been merged and published in downstream modules