Improve and prettify cmake

[gassmoeller]

@tukss: This PR improves and fixes a few issues of the cmake build system. It does:

• remove the MKL_PARALLEL option of the FindMKL script. This option enables linking to the threaded MKL libraries, which is not wanted for Rayleigh, and it causes some issues with discovering the FFTW libraries in MKL (at least on Stampede3)
• enable native optimizations for the new Intel clang based compiler ifx
• allow to install Rayleigh into user provided directories (default is still ${CMAKE_SOURCE_DIR}/bin)
• set the default build type to Debug and output build type and install prefix during configuration
• prettify the cmake output

[gassmoeller]

As discussed with @tukss and @feathern the default is now the optimized mode and we offer all build modes.

For reference, this is how my cmake configuration now looks like: