Add a 'truncate' option to CONV().

`H = CONV(F, G, 'truncate')` will truncate the domain of H so that it is the  same as `F` and `G`. (If the endpoints of the domains of F and G are different then an error is thrown.) 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.