Explicit derivation of functions.

Explicit derivation of functions.

Some function cannot be cleanly differentiated through the chain rule.
For example, it is natural to define derivative of the absolute value as

(abs(f))'=(f'*f.conjugate()+f*f'.conjugate())/2/abs(f)

This patch adds a possibility to define derivatives of functions in this way.
In particular the derivative of abs(), Order(), real_part(), imag_part() and
conjugate() are defined.

For example, conjugate of a derivative with respect of a real symbol
If x is real then U.diff(x)-I*V.diff(x) represents both
conjugate(U+I*V).diff(x) and conjugate((U+I*V).diff(x))
Thus in this patch we use the rule

conjugate(f)'=conjugate(f')

for a derivative with respect to the real symbol.

Signed-off-by: Vladimir V. Kisil <kisilv@maths.leeds.ac.uk>