Integrate HourDayMultiHeatMap with GenericGrid

HourDayMultiHeatMap.js needs to be integrated with GenericGrid.js for the grid management side of things and made consistent with the look/feel of the other visualisations.

HDMHM was written prior to GenericGrid but hasn't yet been refactored. Ideally need to change GenericGrid to allow it to draw one axis per row/column when that axis is synched. This will allow it to work in the same way as HDMHM. Having the axes outside of the cells should probably be a configurable option.

Start by reading HourDayMultiHeatMap.js and GenericGrid.js, then compare the other visualisations to understand their shared look and grid management. Define how GenericGrid should support synchronized axes per row or column and whether axes outside cells are configurable. Done means HourDayMultiHeatMap is integrated with GenericGrid and consistent with the other visualisations.

Written by the indexing model from the issue text.