Different styles of signal aspects

[mb-m]

I was looking into doing different types of signal aspects (eg, in DB - because I live in Germany, signals are RED (double-red), AMBER (amber-green), and GREEN (green)). I had hoped to look at UK too, but the extra double-amber aspect is a bit too big of a change to do - so let's ignore that one for now.

I don't yet have the trains or the signals to test this (I'm just getting into this, but I come more from the software and control systems side than the scenery-building), but I had a look at the RMFT2::doSignal() (EXRAIL2.cpp), and I think that the LED section ends up much cleaner with a sort of LED aspect "map". Given the int16_t for the signal flags, there was only space to add one more flag, so I settled on a main aspect map to 3 LEDs and an alternate, with new EXRAIL macros for SIGNALA and SIGNALAH to use the alternate map. This could also allow shunt signals, I guess.

Interestingly, using an aspect to pin map also makes the code easier to follow (at least from my point of view), because the red/green simulated amber can be handled there, as can the high-inversion of SIGNALH (and the new SIGNALAH).

As I say, this is not yet tested, as I don't yet have all the starter bits for my modelling (including the arduino), but I wanted to share and see if this was the kind of thing anyone else was interested in, if it's the right kind of style (I did clean up a few blank-space parts), and the contributing suggests to open an issue first.