Add support for FHIRPathMappingLanguage data extraction

[Itskiprotich]

Closes #3

[ellykits]

Is this different form TemplateBased extraction? Is htis the one ported from Beda's implementation? Not sure if we really need to support this too, it's community driven and not FHIR standard.

[ellykits]

We can discuss it, let's prioritize the template and definition based extractions.

[Itskiprotich]

Is this different form TemplateBased extraction? Is htis the one ported from Beda's implementation? Not sure if we really need to support this too, it's community driven and not FHIR standard.

That's correct @ellykits, making reference to this clarification here

[LZRS]

@Itskiprotich What's the progress here, or could the PR be marked as draft?