Add help for the formula structure input for non-linear regression

[gkronber]

The information should include a list of all allowed functions, the grammar as well as usage examples.

[gkronber]

Here is the list of operators and functions that are currently (3.3.16) supported by auto diff:

+
/
*
-
ABS
EXP
LOG
SQR
SQRT
CUBE
CUBEROOT
SIN
COS
TAN
TANH
NORM
ERF
AQ 

Power is not supported. There are two work-arounds in 3.3.16:

1. use the pow(x,y) = exp(y log(x)), to optimize both x and y
2. use a combination of sqr and cube for fixed integer powers