Investigate potential CMake improvements

[jake-arkinstall]

At the moment, dependant projects need to find and link Eigen3 in order to link tket::tket, because:

• public-facing headers depend on eigen headers.
• eigen is linked privately in CMakeLists.txt

Ideally, tket/test/CMakeLists.txt should build by linking only tket and catch2, and having cmake transitively include the dependencies of tket.