Broken beams' slopes

[Mike Solomon]

Hey all,

I'll be leaving on vacation in a week-ish, and as my summer-of-lily comes to a 
close, I can likely do one more medium-scale thing before I have to start 
correcting parallel fifths.
I'd like to work on broken beam slopes such that a beam can break across lines 
and pick up where it left off at the same slope and y-offset

The approach would be:

1)  Get rid of the chained callbacks to calculate Beam #'position.
2)  Work on the Beam_scoring_problem class such that it does everything that 
all of the chained callbacks used to do.
3)  Make the calculations in Beam_scoring_problem such that they always unfold 
in a coordinate space where the origin is at the X and Y offset of the first 
normal stem (the latter of which should, aside from really esoteric cases, 
always be 0.0).

This way, it is relatively trivial to tack on information from extra systems 
(or not) - grobs on subsequent systems have their x-displacement added onto the 
combined spanner lengths of previous systems plus the tiny part of the broken 
beam between its left bound and the first stem on a new system.

I estimate that this will take me two days of work, so please respond in the 
next 48ish hours if you think this is a bad idea.  Otherwise, I'm gonna give it 
a go.

Cheers,
MS