calculates *result = fac1 mat1 + fac2 mat2* + ...

The strings *result*, *mat1*, *mat2* are internal names specifying
the matrices. *mat1*, *mat2* must exist, otherwise an error occurs.
If *result* does not exist, it is created.

The factors *fac1*, *fac2* are optional (may be variables). If not
given, one is assumed.

The nuclear values associated to the individual matrices are added accordingly and
the result is associated to *result*.

molpro@molpro.net 2019-06-17