DSS export fixes

[domi4484]

• Remove unexisting get_pk function
• Add missing common functions
• class surface_runoffparameter - new as seperate class and not subclass of catchment_area (see INTERLIS Import test-dataset-DSS.xtf #181 (comment))

Fix #188

[sjib]

@domi4484 I tested the export (dss, kek, sia405 abwasser and selection) - it works fine now except #202

I suggest someone from opengis should review the code and then we can merge this so we can test the other patch "fix length of labels OID suffix and hardcoded shortcut_en"

[sjib]