add func_deriv to custom_1d_model
add func_deriv example to docstring
added func deriv to custom model 1d test...but test fails
doctest doesn't like line break here
line break
Fix to use jacobian instead of function derivative
Clarified docstring on custom_model_1d about the kind of derivative
Merge pull request #1688 from adrn/modeling/func_deriv with manual merge