Some little optmization
closes #3
Added validation for Z elelements
Added Z elements paragraph to tutorial
Removed duplicated assignment of datatype for Field
Implemented direct import of datatypes from a module (e.g. from hl7apy.v2_5 import ST)
Fixed #12. Refactored some test cases
Implemented message profiles (closes #2)
Changed to accept also a file as input
Changed to automatically get the correct profile from the name of the message
Added documentation for message profiles
Merge branch 'message_profiles' into develop
Minor changes in documentation for base datatypes
Renamed profile_parser.py and changed setup to install it as a script file
Moved library description to setup.py