initial changes for #1330
fitters now return a copy of the model
change documentation examples appropriately
make fitter.weights a property
address comments and fix some additional bugs
Slight tweak to how bounds are being passed in to SLSQPFitter, only because for some reason I was getting exceptions like 'TypeError: ufunc 'isinf' not supported for the input types, and the inputs could not be safely coerced to any supported types according to the casting rule ''safe''' which had to do with the fact that bounds was an object array.  Might be related to my Numpy and/or Scipy version.  In any case I think this is a bit simpler since it's taking advantage of the fit parameter indices returned by _model_to_fit_params to select out which bounds to pass in.  Also changed EPS and MAXITER to DEFAULT_* since they can be overridden by __call__ arguments.
Commented out this dead code for now--we'll revisit this obviously
Kept _model_to_fit_params as a ParametricModel method--I'm still not crazy about having it there since the way it adapts the model's parameters for fitting is still more a detail of the fitter implementation.  I kept the reverse operation on fitters so at least there's some two-way adaptation symmetry going on.  Moved _deriv_with_constraints back into LinearLSQFitter since, like with _wrap_deriv, it's very specifically a fitter-specific adaptation of models' derivative functions to the format used by the fitter.
Correct Trapezoid1DModel.eval (fix issue #1721)
Add test for Trapezoid1D model
Merge pull request #1726 from cdeil/issue_1721

Correct Trapezoid1DModel.eval (fix issue #1721)
Updated required Numpy version in rtd-pip-requirements
initial changes for #1330
fitters now return a copy of the model
change documentation examples appropriately
make fitter.weights a property
address comments and fix some additional bugs
resolved conflicts
Merge pull request #1702 from nden/fitter-without-model-ref

remove model reference in fitters