Groundwork for function-point analysis chapter (option --fpa)

The old --FPA option for generating an Excel workbook is renamed to --excel.
Set up basic infrastructure for function-point analysis

(Not integrated with the FSpec yet)
Move FPA computation to FPA.hs and refactor legacy FPA modules
FPA berekening geijkt met Frank Vogelenzang
Change type back to FPType

@stefjoosten: Does the cake rule apply to branches as well? :-)
Cleanup by having fpaDataModel return Maybe

@stefjoosten: FPA.fpaPlugInfo and Fspec2Excel.showDetailsOfPlug contain nice examples of pattern guards.
minor
Cleanup
Merge pull request #5 from AmpersandTarski/add-FPA-chapter

Add FPA chapter