Feature: Improve visualisation and reduction of notes

[pettter]

Maximal expansion, and proper visualisation of reductions

In the current version (6d1b4a22) of the reductive tool, we hide notes that are reduced away, but leave the remaining score in place. This is sufficient to show the successive reductions/generative operations, but is pretty far from the type of visualisation we ideally would like to have.

While we may also in the future also allow for "de-horizontalisations" (see #19), a good start is arguably to at least allow to reduce away "empty time-slices" in a maximal expansion of the score.

There are a number of difficulties that need to be overcome. In particular, If we simply remove the reduced notes from the respective measure/staff/layer, this shifts the other notes within this measure forwards, which may be what we want in some cases, but without augmenting the length of the remaining notes, errors will creep in.

We can access a maximal expansion of the current MEI using Verovios renderToTimemap function, but this only gives us on and offsets, not note presence and duration.

We have basically two possible ways forward:

• We can augment the length of the other notes as we reduce away to preserve note onsets at their proper place, possibly modifying note heads, if necessary
• We can directly construct the maximal expansion by making each slice into a measure, derived from the time map, and remove measure with only held-over notes

[pettter]

For now, we do a smaller version, where we rerender into the old measures just simply removing the hidden/reduced notes.

This ain't pretty, but it works for now. It needs fixing.

[pettter]

Update: the smaller, simpler version is fixed and now works, though the interaction with the MEI and downloading is still a bit unclear.