WIP: Install headers for use by dependent projects

To be able to call into Allen from other projects, for example for Online integration purposes, some headers need to be installed when calling make install.

In addition, the existence of libAllenLib.so (the main shared library produced by the Allen project) must be known when declaring it as a linker library in a package in another project, for example in AllenOnline in MooreOnline (see MooreOnline!65 (closed)) To achieve this in the context of a Gaudi project build, some inspiration was taken from GaudiProjectConfig.cmake. This, however, will likely not work with the migration to the way of doing cmake, so this is WIP until a more permanent solution has been implemented.