Make HourDayMultiHeatMap support different data types, e.g. ordinal

The HourDayMultiHeatMap needs to be made more generic (and renamed to suit) so that the x and y axes can support non time series data, e.g. ordinal values. The best way to do this would probably be to change the input data to have two values, one for x and one for y. The settings json could supply a time format string for datetime data (i.e. for rendering the ms since epoch as a day of the week or an hour of the day. If you wanted the standard day/hour heatmap then you would just supply the event time floored to the nearest hour for the x, and the event time floored to the nearest day for the y. This assumes that Stroom will do all the aggregation and only one value array will be supplied for each heat map cell. Doing all this would mean we can get rid of all the various non-gridded heat map varients.

The issue names HourDayMultiHeatMap and the non-gridded heat-map variants but no files or tests. Start by locating those visualisations and compare their input and settings handling. Before implementation, clarify the generic x/y data shape, datetime formatting, aggregation assumptions, and replacement scope.

Written by the indexing model from the issue text.