Heatmap assumes uniform spacing of patterns

[ml-evs]

e.g., if I remove the middle 50 patterns from a run, the heatmap just gets stretched to the same end time but shows the other patterns without any discontinuity.

[jdbocarsly]

Handling this properly would also make it easy to change the y-axis of the heatmap to battery capacity (or even mols of Li transferred), which would be very helpful. In some cases this is a straightforward transformation from time, but in cases of voltage holds, etc., it will lead to non-uniform spacing of the patterns and require interpolating onto a regular grid to make the heatmap.

[ml-evs]

In #78 I've added waterfall plots which could be used in this case too