3226: give diff a simplify option
make high-order derivatives simplified by default

Only factor_terms and signsimp are used in the simplification; other
simplification may be possible but that is left for the user
to decide if it's necessary.

* do not simplify in euler_maclaurin

For some reason the size of the derivatives grows if the
simplification is used. I don't know why.
Merge pull request #1998 from smichr/3226

3226: give diff a simplify option