refactor divideByReference()

[cvanderaa]

I think we should consider several improvements:

1. divideByReference() is a type of normalization (normalization by reference channel), therefore I think it should be made accessible from normalizeSCP(). This means we need to implement a new method in MsCoreUtils.
2. Since it is a type of normalization, a new assay should be created instead of replacing the target assay.
3. We could make it more generic, including subtraction as well (in case of normalization on log-transformed data).

[cvanderaa]

(point 1.): Strictly speaking, dividing all multiplexed samples within a run with a reference channel is batch correction rather than normalization (though many call it normalization anyway).