Integer variables in equations

Ok, this is one is much more straightforward then thinking about references :) It looks like a lot of changes, but most of them are the renaming of `default_scalar_dtype` to `default_float_dtype` and the removal of the explicit `is_bool` argument. There are some subtleties surrounding integers that we should address at some point (e.g. with numpy, defining a variable as `int16` and then using `G.var = 'floor(x)'` will not work since `floor` always returns an `int32`) but I think for now this is good enough.

From my side: ready to merge