Need a test for demos

[robbietuk]

It appears that there isnt a travis configuration that tests the C++ demos, even building them, as discovered in #888.

[robbietuk]

Testing the demos with travis would be useful for #896 Two step process:

1. Have a travis job build with STIR_LOCAL, e.g. add the cmake flag -DSTIR_LOCAL=$STIRSOURCEDIR/examples/src . It is not clear how to get the $STIRSOURCEDIR as travis does not currently use it.
2. Run the demos in testing. These are all low computational effort so will not dramatically increase build time. However, the parameter files should probably be installed into $TRAVIS_BUILD_DIR/src/examples/C++