Several EvtGen model enhancements and a fix

[eduardo-rodrigues]

Several matters:

1. Continuing further on Test suite to verify all EvtGen models are parsed correctly - take 8 #394 and similar PRs to deal with task Enhance test suite to verify all EvtGen models #357. Closes Enhance test suite to verify all EvtGen models #357.
2. Adds LHCb model RareLbToLll.
3. Removes model LQCD wrongly added in PR More EvtGen models and keywords (mostly for LHCb) #336. LQCD rather is a model parameter.
4. Enhances EvtGen model parsing to be able to cope with XML file paths given as model parameters.
5. Enhances EvtGen model parsing to be able to understand negative model parameters defined via a Define statement, e.g. with a Define blabla ... and then a model line1.0 <list of particles> <model> -blabla.