Removed hack where the bed PWM value was inverted (as my previous bed driver was inverting the signal)
Added config options and descriptions to enable look-ahead in Gen7-v1.1-v1.3, added some sanity tests to dda.c and made look-ahead less verbose.
Split most lookahead logic into seperate file and added runtime configuration.

This cleans up dda.c a lot and groups most of the needed logic for lookahead into a single file.
To aid debugging and evaluation (and perhaps to maintain), the logic is made runtime configable by means of 3 new M codes.
M141: Enable look-ahead optimisation (quiet)
M142: Disable look-ahead optimisation (quiet)
M143: Print look-ahead status. This includes some counters to show how it is performing.