Clean up, part 1 (read longer commit message!)

- streamlined fSRM function
- removed unnecessary code
- added update.fSRM function
- the Cook2000.R file now contains some sort of unit test: all major
scenarios are tested

- renamed return values of fSRM:
  - the lavaan model is called "fit" (formerly: "res")
  - the syntax is called "syntax" (formerly: "model")

Still to be done:
- Rewrite print function
- find bug in mean structure calculation