Arithmetic operators on a measure A are only defined
in some cases. The arithmetic operator is effectively applied
to the value of A(W) for every spatial domain W.
If the result is a measure, then this operation is valid.
If A is a measure (object of class "msr")
then the operations -A and +A are defined.
If A and B are measures,
then A + B and A - B are defined.
(This is implemented only when A and B
are represented using the same quadrature scheme.)
If A is a measure and z is a numeric value,
then A * z and A / z are defined,
and z * A is defined.