Has the following functionalities:
- `StackedSparseArray` stores one or multiple sparse arrays in a "stacked" manner. This means it keeps track of one set of 2D coordinates (row + col) and a stack of values for all row, col positions.
- More data can be added in coo-format using different merge/join-types (e.g. "left" or "inner").
- Allows several slicing options.
- Can output data as dense array (structured numpy array).