Harmonization of Image and HeatMap

[flying-sheep]

There are a few issue trying one or the other depending on

• categorical vs continuous coordinates (Image doesn’t support the former)
• density of data (HeatMap can show ugly 1-pixel gaps and be slow)

@philippjfr said this should be fixed to allow seamless use of all coordinate types.

See here for details holoviz-topics/hv-anndata#82 (comment)