Misc Model improvements...

[danielballan]

• API: N=Parameter(value=7) is equivalent to N=Parameter(name='N', value=7).
• CLN/PERF: Build residual only once, on instantiation.
• ENH: Allow models with no independent_vars.
• ENH: Support weighted fits.
• BUG: Copy parameters on input.
• ENH: Handle missing data.
• BUG: Accept pandas.Series equivalently to numpy.ndarray.

[danielballan]

Here's a link to the updated notebook on nbviewer (updated with plotting from commit below)

[danielballan]

After tagging on those few commits to make the feature set more complete and consistent, I think this is ready to merge.

[newville]

Great! Thanks... Today's been a bit busy for me, so I'll try to comment on
many of the suggestions here in one email:

a) I completely understand not wanting to get too over-committed.
b) I like your suggestion of sublcassing from Model and being able to
"add Models".
c) I've never really loved the "scikits.something" names, but I think we
need a github organization. I think "lmfit" is a little too narrow for
much of what is happening here (better Models, Error Analysis, and so on),
but is "lmfit" OK as an organization name? Perhaps we could start with
that and work on a better name?

--Matt