SLSQPFitter tries to fit all parameters

The `FItter` classes have an internal attribute called `Fitter._fitpars` which is a list of all parameters from the model being fitted *excluding* tied and fixed parameters.

So for example if a model has 3 parameters, one of which is fixed, a fitter for that model has only 2 parameters in its `_fitpars` list.

However, in the `SLSQPFitter`, all of the model's parameters were being used.  When the fitter goes to set its `_fitpars` with newly fitted values after a pass through the `fmin_slsqp` it changes the size of `_fitpars`.  This doesn't break anything obviously, but it can lead to inconsistent and incorrect results, I think.  @nden thinks this is a bug too.