Show a "codish" overview of a FHIR mapping

[MiskoG]

I had this idea after seeing this work by @lnmylos : https://github.com/arkhn/ops-mimic/pull/27

The idea is to have a "code" summary of a single mapping.
This is read-only, meaning that we cannot edit a mapping through this code. It is pure display.
This means also that we can be a bit more "chill" on the rules of this new mapping syntax, since it won't be compiled or used by a computer for now.

Goals :

• give a convenient overview of a mapping inside Pyrog (currently, you have to go to each individual attribute node)
• slowly experiment on the interest we could have for a "programmatic" approach of a FHIR mapping

cc @elsiehoffet-94 @nriss @lnmylos @Peltier10

[nriss]

I also worked on a quick mapping-to-FML converter that could be shown in this page
https://github.com/arkhn/FhirMappingLanguage/pull/9

[MiskoG]

FYI @simonvadee @Jasopaum @tevariou : an other possible approach for the FHIR mapping language!