Make FTorch REQUIRED in the examples

[jatkinson1000]

It came up in #124 that some linking could pass undetected for the examples as CMake does not currently 'REQUIRE' FTorch.

This could lead to linking issues later on at link or runtime that should be detected earlier when building.

We should update the examples (which are then pulled into the tests) and the online coupling guidance.

[jatkinson1000]

Discussion with @jwallwork23 we think this would require making the building of tests a separate step from building of the library, but doing so would not be the end of the world.

This is not a priority, however, so we can stall this for now.