fix cmake
The cmake correctly export MRF headers and lib when installing.
A findMRF.cmake is now provided in MRF/cmake to locate MRF headers and lib if MRF is installed, using the cmake find_package(MRF)
routine. The findMRF look in default install dir, otherwise it must be set.
Also, reworked a bit some of the folder hierarchy to be more explicit and consistent.