- Support coercing from `XcmsExperiment` to `XCMSnExp` using
  `setAs(object, "XCMSnExp")` (issue #752).

- Change `estimatePrecursorIntensity()` to a method and add an implementation
  for `MsExperiment`.

- `chromatogram` with `msLevel = 2L` does by default extract chromatographic
  data of all MS2 spectra, independently of whether different m/z isolation
  windows were used. For data with isolation windows, the respective isolation
  window can be set/defined with the `isolationWindowTargetMz` parameter.
- Update documentation to describe this behaviour better.