getMeasurementXY is too loose

https://github.com/cheminfo/analysis/blob/274c2a7326a3046be976d6bd5e3613ad09d00e3e/packages/base-analysis/src/util/getMeasurementXY.ts#L26

There are a lot of cases where the function can return `undefined` and then this pollutes the [Analysis class](https://github.com/cheminfo/analysis/blob/274c2a7326a3046be976d6bd5e3613ad09d00e3e/packages/base-analysis/src/Analysis.ts) where most methods can also return `undefined` and the user always has to check if a value was returned.