Added cereal include directory to CMake

In preparation for the serialisation - and declared it empty, to work around a VS bug with =default

Removed all non-header-only CMake stuff and added a custom target for the library headers so they show up in IDEs

Fixed PcaModel c'tor, forgot to copy the arguments

Expects an isomap to exist with the filename <imagename>.isomap.png.

Added eos-config, -config-version and -targets CMake scripts so that the library can be included by other CMake projects