Add functional test for loading simple example model

[alexdewar]

While we are checking whether the various routines for loading input file data with unit tests, it would also be useful to have functional tests to check that the example models load too. This would give us test coverage for some of the higher-level functions, as well as providing a sanity check that the example models all work (i.e. that there aren't mistakes in the model definitions either).

[alexdewar]

I forgot, but this has actually already been done by @Ashmit8583 in #80.