Use PyPNG instead of pyplot for writing PNG
Use PyPNG instead of pyplot for writing PNG

It is unnecessary to use matplotlib just for encoding an existing image
into png. pyplot requires a graphical desktop which we want to avoid.

With this change, a dependency on PyPNG
(https://github.com/drj11/pypng/) is introduced.

SpatialMetabolomics/SM_distributed,SpatialMetabolomics/SM_distributed,SpatialMetabolomics/SM_distributed,SpatialMetabolomics/SM_distributed,METASPACE2020/sm-engine,METASPACE2020/sm-engine