Allow `cim()` to take `block.spls` object

[Max-Bladen]

Is your feature request related to a problem? Please describe. A block.spls object cannot be passed into cim(), despite the fact that the functionality already exists to produce a CIM for spls type objects.

Describe the solution you'd like Utilise the framework that already exists for using cim() on a spls object, but just ensure that there is a parameter which includes which blocks should be used, as a maximum of two can be used to produce a clustered image map.