Request: Expose #define for "BPD" in pigpio.h

[ahmogit]

The numerical value of BPD (presently #defined at pigpio.c::790 in pigpio V79) is necessary to have available if one wishes to perform dynamic adjustments to wave segment lengths. However it is not presently available as a preprocessor symbol in pigpio.h, leading to inscrutable hardwired "magic number" usages like this

 `*g_slackA = (slack * 4);`

as appears in example program pps.c::189. Having BPD available in pigpio.h would avoid the magic number 4.

Regardless of whether one agrees or not that BPD ought to be exposed in pigpio.h, it would certainly be useful to add a trailing comment to the #define for BPD, stating its semantic. I guessed that it probably stands for "bytes per [something]", but not sure what [something] is.