[BBPBGLIB-1132]Replace StimulusManager.hoc with the python classes (#132

)

## Context
Deprecate the hoc implementation of the stimulus manager.

## Scope
Replace StimulusManager.hoc by the exsiting stimulus_manager.py where
all the stimuli have already been defined in the dedicated python
classes, except the type `extracellular_stimulation` (pending for
BBPBGLIB-890).

## Testing
New unit test in `test_sonata_config.py` for parsing "subthreshold"
stimulus.

## Review
* [x] PR description is complete
* [x] Coding style (imports, function length, New functions, classes or
files) are good
* [x] Unit/Scientific test added
* [ ] Updated Readme, in-code, developer documentation