Adding Pin Colors to Dimensionality Reduction Plot

[LizaShch]

Summary:

Updated dimensionality reduction plot to take in pin_colors keyword argument which allows users to pass in a string of a key located in ann-data uns. It then grabs the dictionary/color mapping and passes into 2dscatter function.

Changes:

• Adds color_mapping string keyword
• Changes the ordering of coloring for the 2dscatter, using color_map as an if/else condition.
• Accidentally added the changes from scatter into this one due to changing keyword condition.