Move documentation to read the docs

[edoddridge]

This PR moves documentation building and hosting to read the docs.

As described in #156 there are a number of advantages to moving the documentation to read the docs.

It also implements a couple of house keeping changes:

• figures are stored near the simulations that produced them, rather than the docs folder
• the benchmarking figure uses a different marker for Hypre and the internal solver, so that the difference is clearer in B&W and for colourblind people.

It also stops Travis installing valgrind. The sudo apt-get install valgrind command was throwing an error. Rather than investigating it in detail, the command was flushed from .travis.yml since valgrind was no longer being used.

[codecov[bot]]

Codecov Report

Merging #157 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #157   +/-   ##
=======================================
  Coverage   94.09%   94.09%           
=======================================
  Files           5        5           
  Lines        1134     1134           
  Branches       57       57           
=======================================
  Hits         1067     1067           
  Misses         52       52           
  Partials       15       15

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