Fix hack in Disturb_rate function by using exact step function

[akutuva21]

Replaced the steep sigmoid approximation in the Disturb_rate() function with a true step function using a BNGL if statement: if(t > disturb_time, disturb_mag, 0). Tested via bionetgen run to verify that the cvode solver smoothly handles the discontinuous function without failing. Also removed the obsolete comment about the hack.

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PR created automatically by Jules for task 541052412947067788 started by @akutuva21

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