wrap up an atlas object from mc, mc2d and matrix ids

This is not doing much more than generating a list with the relevant object names bundeled. To be enhnaced at some stage.

Arguments

mat_id

id of metacell object ina scdb

mc_id

id of metacell object ina scdb

gset_id

features defining the atlas (to be sued for determing projection)

mc2d_id

projection object id, to define the atlas 2D layout