Add scale parameter to SampleFromUniform

[sethaxen]

SampleFromUniform initializes all parameters uniformly in the range [-2, 2]. Sometimes it is helpful to initialize over a wider range. RStan offers the init_r parameter for increasing or decreasing the bounds, and it would be useful to have that here as well.

What if SampleFromUniform had a field like upper or scale, defaulting to 2?

[yebai]

It looks like a very sensible improvement/feature.

[mhauru]

@AoifeHughes, another issue that requires touring DynamicPPL a bit but not necessarily understanding deeply what everything in there does.

[penelopeysm]

This would actually be pretty useful 👍

[penelopeysm]

Closed by #981