Set up doctests to catch errors in the examples

[CPBridge]

Various mistakes have been found in the user guide examples. Examples that don't work are very offputting for new users. We should set up automatic testing of the documentation examples to ensure they run correctly.

[CPBridge]

Doctests has been set up in #165 to catch errors in docstrings.

However this does not catch errors in the RST examples, so leaving this open