Separate out filter functions from peak finding in blob_dog, blob_log

[jmetz]

Description

It might be useful to have access to the "image cube" filter data generated in blob_log and blob_dog, plus separating the generation of these into dedicated functions could afford a more suitable level of modularity.

I would propose two new functions in the filters module, named e.g. scale_space_log and scale_space_dog or similar, and then calling these in the blob_dog and blob_log functions.

[bernwang]

Where is the filters modules should these new functions be? Should they be in an existing file or a new one?

[jmetz]

My suggestion would be to have a scale_space.py file in filters - but I guess we should wait for a core dev to weigh in.