Enrichment Visualizations and Visium Polygon Creation

[mattobny]

Added:

• New File spatial_enrichment_visuals.R
• New function findCellTypesFromEnrichment() within spatial_enrichment_visuals.R to show most probable cell types based on a provided enrichment
• New function plotCellTypesFromEnrichment() within spatial_enrichment_visuals.R that generates a bar plot of cell types vs frequency based on a provided enrichment
• New function pieCellTypesFromEnrichment() within spatial_enrichment_visuals.R that generates a pie chart of cell types based on a provided enrichment
• New function addVisiumPolygons() within convenience.R (along with its requisite internal functions) that adds circular polygons centered at the spatial locations of a Giotto Object made with Visium data. Takes a Giotto Object and a path to the Visium output file scalefactors_json.json as input arguments.
• Added addVisiumPolygons() to createGiottoVisiumObject() workflow.

NOTE: Highly advisable to update the installation of GiottoClass to this PR when this update is pulled. This update to GiottoClass ensures addVisiumPolygons() assigns polygon centroids as close as possible to the spatial locations.