tweak field selection

[westram]

Goals:

• ensure called algorithm and selected field fit together finished with [14779]
  • in some cases it's allowed to select a numeric input- or output-field, but the corresponding algorithm is not capable to read or write fields of that type

• provide simpler interface for interaction with -selectable fields: [14699]
  • algorithm knows what datatype it wants to write (e.g. BYTE, INT, FLOAT, STRING, …)
  • field selection shall use that datatype and provide the matching fields

• allow to create fields on-the-fly [14721]

• fix macro problems of field selection [14750]

• some field selections use an inputfield (e.g. "Create merged species..") because they intend to write to new fields only [14781]

[westram]

by [14786]