Ensure increment_var value is a Number

[watsonian]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	#4965
License	MIT

This ensures the increment_var value is coerced into a Number before attempting to perform any math operations on it. Doing so solves an issue where when you pass in a PORT variable through the environment rather than through the ecosystem config file, it would be read as a string and incrementing it would append the number to the string rather than actually incrementing it (e.g., 3000 would result in using ports 3000 and 30001).

[CLAassistant]

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