StimulusFactory: enable amplitude based versions of the protocols

[anilbey]

At the moment the following protocols are only threshold-based.

• neg_cheops
• pos_cheops
• fire_pattern
• iv
• idrest
• ap_waveform

Some of the non-bbp models however are amplitude based.

The StimulusFactory should be extended to support the amplitude based methods as well.
The amplitude based implementation can be reused by the threshold based implementation.

Such as the following:

BlueCelluLab/bluecellulab/stimulus/factory.py

Lines 264 to 272 in fcf8ad0

	amplitude = threshold_current * threshold_percentage / 100
	res = cls.amplitude_based(
	dt,
	pre_delay=pre_delay,
	duration=duration,
	post_delay=post_delay,
	amplitude=amplitude,
	)
	return res

[AurelienJaquier]

We can now use amplitude based (i.e. non threshold based) stimuli in apply_multiple_stimuli by passing raw amplitudes to amplitudes and setting threshold_based to False.
pinging @stefanoantonel because he might need to inject without using threshold current.

[stefanoantonel]

Cool thanks!!