Manually-specified paths to override automatical detection

The regex will only be applied to the environmental variable PUDIR if it's defined and on Windows, thus prevent issues when it's not defined.

PU_INCLUDE_DIR and PU_LIBRARY_DIR cmake variables are added to let users specify the paths to their ParticleUniverse headers and library respectively. This makes it possible to detect PU even when it's not in a conventional installation directory, or when you want to use a different version than the one installed on your system.