Add subthreshold stimulus support

[darshanmandge]

• Solves Add SONATA support for subthreshold stimulus module #79.
• Add subtreshold protocol as described in sonata-extensions.https://sonata-extension.readthedocs.io/en/latest/sonata_simulation.html#subthreshold-current-clamp

[codecov]

Codecov Report

❌ Patch coverage is 97.22222% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
bluecellulab/circuit_simulation.py	66.66%	2 Missing ⚠️

Files with missing lines	Coverage Δ
bluecellulab/cell/injector.py	94.40% <100.00%> (+0.18%)	⬆️
...ecellulab/stimulus/circuit_stimulus_definitions.py	88.16% <100.00%> (+10.16%)	⬆️
tests/test_cell/test_injector.py	98.75% <100.00%> (+0.02%)	⬆️
...s/test_stimulus/test_circuit_simulation_stimuli.py	99.45% <100.00%> (+0.05%)	⬆️
...test_stimulus/test_circuit_stimulus_definitions.py	100.00% <100.00%> (ø)
bluecellulab/circuit_simulation.py	84.75% <66.66%> (+0.52%)	⬆️

... and 6 files with indirect coverage changes

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