Improve FitResults

[pavoljuhas]

Proposed updates:

• remove recipe attribute to make FitResults pickleable
• add dictionary-like access to FitResults values
• add items() - like generator for FitResults.

[ycexiao]

@sbillinge, it can not be implemented quickly. I think we can put it in the next release.

In many places, FitResults takes recipe as an input. And some methods in FitResults depend on self.recipe. We need to refactor a lot of places to close this issue.