Use underscore suffix instead of prefix for primitive #6

[ruscoder]

Within this PR I've changed the underscore prefix to the underscore suffix. Moreover, in the code, it was already used for keywords like for_, class_. See my last comment #6 for details.

Also, I've overridden the base model class changing dict() behavior to return the alias name by default. (Closes #17)

[ruscoder]

Having _ is not enough because it overlaps with reserved keywords, e.g. import_: str and import_: Element. So, I decided to adhere to fhir.resources python library where it's exposed as __ext suffix.

[ruscoder]

I clean up the code by merging into main small pieces:

• header.py.tpl
• fixes for Reserved keywords are not dumped by dict by default #17 and Unable to initiate resource with reserved keyword for/class/import #19
  Now the code contains only changes related to primitives