user can now use all available character and factor variables for stratification and filtering except for the _id variables
check for the outbreak_status column values which we have predefined for the algorithm evaluation will still be added when working on #286
the "only" danger is that now the user has much more possibiities for stratifications which might not even make sense so it is more in his/her responsibility to understand thus there might be more explanations/training needed
resolves #268
user can now use all available character and factor variables for stratification and filtering except for the _id variables
check for the outbreak_status column values which we have predefined for the algorithm evaluation will still be added when working on #286
the "only" danger is that now the user has much more possibiities for stratifications which might not even make sense so it is more in his/her responsibility to understand thus there might be more explanations/training needed