improving interface...
more improvements and cleanup
further improvements, but at the moment it is BROKEN
error checks for params and entities
lots of checks to catch errors early to provide a good message and the correct stacktrace
use exceptions for all errors
We have entity classes for all entities - only constraints are missing
some constraints
two more constraints; now we have all constraints that are used by the test

I'm a bit hesitant to implement the other ones because I'd have to guess the parameters
and I don't want to mislead anyone.
generic constraint constructor that can be used, if a constraint implementation is missing

A lot of constraint types ARE missing because I cannot find any authoritative
information about how they should be used.
add SolidPython submodule
Param and Point can be passed to SolidPython (which will convert them to a double resp. a list of doubles)