Dynamics context spacing issues

[Neil Puttock]

Hi Joe,

I've been testing the new Dynamics context in various situations, and
it's a struggle to get consistently centred dynamics without
trial-and-error tweaks for inter-staff-spacing.  It also seems to be
incompatible with alignment-distances, since the default settings have
no effect in this case (the dynamics line behaves as if it has
staff-affinity = #UP).

Whilst trying to work out some way to make the centring automated, I
had a look at your latest commit, and it got me thinking: if the
staff-affinity = #CENTER, then the loose line has no bias for a stave
above or below, so it seems a no-brainer to allow non-affinity-spacing
to work in such a case rather than relying on inter-staff-spacing.
Does this sound sensible?

Regards,
Neil