Hanfor req-exporter allows export of broken requirements

[Langenfeld]

Hanfor allows for a number of broken requirements to be exported without warning (leading to the ultimte requirements parser being unable to parse these requirements). There should be a warning or at least no export of the following ill formed requirements:

• CONST someConstant IS None, where no value for a constant was given in Hanfor, thus python None was written in the requirement
• CONST some.Constant IS 4, where the variable name contains a . wich is not intended for names in BoogiePL
• Requirements that have non-constant variables in their time bounds
• Requirements that use enumerators of an enumeration that is unused: the enumerators are not defined as constants if the enumeration is not used in the requirements thus the enumerators are unknown in the Boogie file.
• Export while there are variables typed as unknown that are used in the req.