Improve convolution support

[kcrisman]

We have convolution scattered in several places, including

sage.rings.polynomial.convolution
sage.functions.piecewise_old.PiecewisePolynomial.convolution
sage.functions.piecewise.PiecewiseFunction.EvaluationMethods.convolution
sage.gsl.dft.IndexedSequence.convolution

This should be extended to make it easier to use/find and to support more arbitrary inputs, both discrete and continuous.

See http://groups.google.com/group/sage-support/browse_thread/thread/7f90c228df9530dd for background.

Depends on #14801

Component: symbolics

Keywords: convolution, convolve, discrete, continuous

Issue created by migration from https://trac.sagemath.org/ticket/8994

[ppurka]

Dependencies: #14801

[mkoeppe]

comment:6

Updated description to reflect changes in new piecewise implementation (#14801).
Changing to "wishlist" because nobody seems to have worked on it in 6 years