Removed old synthesise method and fixed background in the new version

[lmauviard]

I realized the old synthesize method were not removed.

I also noticed a few issues while synthesizing some data:

1. In synthesise_expo, the compute_expected_counts function was given the background in counts and not in counts/s as required.
2. In the Signal.synthesize, the save background data was in counts/s, although it should have been in counts.
3. Case matching for writing in Signal was using the data phases as reference, and not the input count shape.

All of the above have been fixed