Add a 'same' option to CONV().

`H = CONV(F, G, 'same')` will truncate the domain of `H` so that it is the  same as `F`. This is useful when `F` and `G` represent rapidly decaying functions on large but finite intervals which are used to approximate infinity.

I had a request from a user for this feature. It will also be useful to @ajt60gaibb and myself for investigating convolutions on unbounded domains.