Uses the parameters array to create XML data for parameter settings on
each channel when saving.

This only works for processors that use the parameters array, though I
updated LFP and Signal Generator to update that in parallel (LFP seems
to have an unrelated bug in dealing with channels that is causing some
problems). Eventually, all parameters should be used in the parameters
array, since it opens up a lot of possible holes when using default
values (they need to be specified twice), and once loading is
implemented. I've fixed some of the problems with parameters, so this
should be easier to do.

As part of this, I also fixed the implementation of
addParameterEditors. The old way of implementing a custom parameter
editor was to rewrite the virtual function, but because it was called
in the constructor, this didn't work (constructors use the type of the
parent). Instead, all editors now also pass a bool
"useDefaultParameterEditors" which is used by addParameterEditors() to
determine whether to do anything. I think this is the best way to do
it. It's initialized to default as true, though I had some trouble with
it implicitly taking the argument and so had to add true to most of the
editor functions (which is probably good practice anyway).

I also fixed some of the functions for generic processor to get
parameter index and parameter name.

Obviously, this is of considerable less use without functions for
loading parameters, but I figured it'd be better to make available for
testing with just the loading in case the changes to Parameters broke
anything I missed.