Testplan: a durationBinOp function is needed in order to be able to sum/substract durations

For the time being is not possible to execute some operations of deducting/adding one duration from/to another.
Becides, using the the tolerance is also impossible.

For exmpl.: we'd like to have the following:
(checking that 2 durations are withing +-2 seconds of each other)
Checkpoint eqDuration %Cmd.TimeDifference% %SOAP:Message.SendIn% 2s

Failing this, we could do this:

set %Cmds:SendInLowerBound% as durationBinOp %SOAP:Message.SendIn% - 2s
set %Cmds:SendInUpperBound% as durationBinOp %SOAP:Message.SendIn% + 2s
Checkpoint durationComp %Cmd.TimeDifference% < %Cmds:SendInUpperBound%
Checkpoint durationComp %Cmd.TimeDifference% > %Cmds:SendInLowerBound%

We need some sort of way out, at the moment none is possible.

In this case we can save us the four abobe lines and it would be much more readable.