Improve scaling for message-/field-definitions

[J0EK3R]

Till now, the scaling capability of fields is limited to a single divide- or multiply-operation by setting a positive number as divisor or negative number as multiplicator.

e.g. division

16_Gradient10,SIN,10,,Steilheit

e.g. multiplication

_8_Rpm60,UCH,-60,Rpm,Drehzahl

In my Weishaupt-System there are parameters like "timespan since last command" wich holds the timespan-value since the last command from the controller of a heating-circuit was sent. (No command in a well known timespan results in an error).
In this case, the timespan value is 300 für 0 minutes, 299 for 0.5 minutes, 288 for 1 minute and so on.

To get the desired result in minutes, a certain scaling operation has to be done:

result = (rawvalue - 300) / -2

It would be very helpful to be able to define this scaling-operation in message/field-definition.
My suggestion - to be backward compatible - is the use of named parameters like:
num - numerator
den - denominator
off - offset

_6_TimeSinceLastCommand,BI0:6,num=-1;den=2;off=-300,Min,Zeit seit letztem Kommando

or the use of a num-den-off-triple in brackets

_6_TimeSinceLastCommand,BI0:6,[-1,2,-300],Min,Zeit seit letztem Kommando