Time derivatives of NE_SW (NE_SW1, ...)

[abhisrkckl]

Complements #1854

[dlakaplan]

Maybe clarify that you are putting in time derivatives here (and not partial derivatives needed for fitting)?

[kerrm]

Two quick comments:
(1) perhaps add a short circuit for non time dependent cases to avoid any performance regression
(2) the test case should include a test of the correctness, e.g. that NE_SW1 = 1 produces a value of 5 a year 0 and 10 at year 5

[abhisrkckl]

Two quick comments:
(1) perhaps add a short circuit for non time dependent cases to avoid any performance regression
(2) the test case should include a test of the correctness, e.g. that NE_SW1 = 1 produces a value of 5 a year 0 and 10 at year 5

Added both.

[abhisrkckl]

I think this is ready to merge.