improved performance has several new options

<ul>
<li>For the overflow buffering, which aims to reduce cylical spillover from the FFT convolution,
a choice of three algorithms is now provided:
<ol>
<li> Extend the p.d.f. somewhat beyond its original domain (the new default)
<li> Fill the buffer 50/50 with the value of the p.d.f at the upper/lower bound of the convolution observable (the previous default)
<li> Mirror the p.d.f. over the boundary
</ol>

The new default algorithm provides a more sensible resu