General `_run` for first order decay about qsdsan HOT 6 CLOSED

Also, this class in placed under the sanunit dir but not a SanUnit, might be better to make it a Process

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Maybe make a Kinetics class for 0/1/2 order kinetics that can be analytically solved, but the name Kinetics is taken now... need to talk to Joy and see if we can change that to Rate or similar

from qsdsan.

Also, this class in placed under the sanunit dir but not a SanUnit, might be better to make it a Process

This has been moved to processes, but it's not a Process either

from qsdsan.

Maybe make a Kinetics class for 0/1/2 order kinetics that can be analytically solved, but the name Kinetics is taken now... need to talk to Joy and see if we can change that to Rate or similar

See #84

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This should have been completed for all public units upon 13868fc, all tests for the corresponding systems can pass (except for a minor change in QSD-Group/EXPOsan@45b59e3 because of more consistent account of N degradation)

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And also done for the private repo as well https://github.com/QSD-Group/QSDsan-private/commit/64d645186a38695208d711f22839a8f59e8e3cbf, can finally close it!

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