Refactor FHIR search

Enable to ways of constructing a FHIR search query, one by chaining FHIRSearchElement instances (RENAMED from FHIRSearchParam!!), the other by supplying a search query dict, like in fhir.js, to a FHIRSearch instantiation. Far from perfect and the two approaches are not (yet?) mixable. Tests are included at the end of FHIRSearch.py and the generated (!) FHIRSearchElement.py files for convenience. Should become unittest files at one point.