RT #includes cleanup

Note that this merge request is not aimed into the master branch.

Hi TK

I looked into why the RT_IMPLEMENTATION macro wasn't working for your branch, and tried tried to see what was going wrong. Turns out there were two issues: You had a convoluted way of including headers, and sometimes the compiler preferred to include the src/rt/SPHM1RT/rt_properties.h file instead of the src/rt_properties.h file with an #include "rt_properties.h", which is a bit strange. For the latter reason, I think it's safer to just keep the macro definition in each RT scheme's individual rt_properties.h file instead of the global one. The alternative is to go down into autotools's workings and setup, which I'd rather avoid at this time given that a pragmatical solution is at hand.

While trying to figure things out, I started refactoring your code a bit to make the header inclusion tree a bit simpler to follow and to disentangle the files. I only moved around the current state of the code, and added a couple of new files to keep inclusions disentangled. I also used the opportunity to do the same for my own schemes. This is the final version I've gotten, I tried to compile all 4 available RT schemes with it, it compiles without a hitch for me.

Let me know what you think.

Best, Mladen