isolate parts needed for `cellpy-core`

[jepegit]

The goal is to provide a slim cellpy-core version of cellpy that only performs the basic operations.

• receive data in a (not yet decided) specified format
• perform generation of a frame-agnostic step table
• perform summary statistics (frame-agnostic, unit-agnostic)

Since the 'full' cellpy package should reuse much of the code from the current version, it makes sense to start to try to distil out a 'core' from the current version (into another module) and make sure the 'current' cellpy works with that core.