Add color schemes to Mod Fill visualizer

[gwhitney]

Currently, Mod Fill simply colors a cell black if anywhere in the series there is a representative of the corresponding congruence class. You can watch the diagram fill in to get some sense of when in the sequence a given residue occurs, but there is no indication in the final diagram.

This binary scheme is reasonable and should be kept, but I don't think it would be overcomplicating the visualizer to offer a couple more color schemes. The ones that come to mind (a subset or superset of these could be implemented) involve the user choosing a gradient (from some standard options like greyscale or typical heatmap gradients) and then using it to indicate one of

1. The position of the first term where that residue occurs
2. The position of the latest term visited where that residue occurs
3. The absolute number of times that residue occurs (not sure if this one makes sense)
4. The fraction of the visited entries in which that residue occurs

In (4) there is the question of whether the gradient should have the same dynamic range for different moduli -- for higher moduli, we might expect that all individual residues have low(er) frequencies, since there are so many residues, so we might want to (automatically, or under user control?) "turn the gain up" on the larger moduli.

This proposal is after an idea floated in #49.

[gwhitney]

Cf #210.

[gwhitney]

These things should be considered but optional for the ModFill overhaul for alpha.