Refactor `PowerFitter`

[BSchilperoort]

The PowerFitter class orchestrates the scans on CPU and GPU.

A refactor of this class is required, to allow for new functionality such as;

• multiple OpenCL queues on GPU (see Bad performance due to many small kernels #51 )
• re-using the same target and only changing templates (Optimized run on multiple templates #60 )

Related issue: #63

[BSchilperoort]

We should also re-add tests after refactoring PowerFitter.
E.g. a regression test that ensures same results are achieved on certain templates/targets