Fix Maven issues

I change the dependencies of this project by adding exclusions to some
dependencies which cause problems with multiple XML APIs.

Furthermore, I try to add support for documentation generation via
Grails but this does not work.
Property editor for numbers

I add a class which acts as property editor for numbers.  This editor
converts strings to numbers and vice versa.  It produces zero for all
empty strings and null.  I register the property editor for all
primitive number data types so it converts string values correctly to
non-null primitive number types.

Furthermore, I revise the existing DatePropertyEditor and write unit
tests for both the property editor classes.
Use double for floating point numbers

I change all domain classes to use double as type for floating point
numbers especially for currencies.  Some properties which also store
null as value for "not defined" use Double as type.  Additionally I
edit all database migration scripts to reflect the type changes.  The
database migration script which adds more fraction digits to all
currency values is removed.

Furthermore, I revise all domain classes and remove the "nullable:
false" constraint because it is default.