Update REST.php

[ThomasLandauer]

• I created a new header "JSONPath", and moved the general JSONPath infos from grabDataFromResponseByJsonPath to it - in preparation for removing flow/jsonpath as a dependency, see Abandoned dependency #24 (comment)
• I removed the "Parts" header, cause it seemed useless to me

[Naktibalda]

Parts header is not useless, it could be clearer, but, I think, the assumption is that the reader is familiar with parts functionality from some other documentation page.

part: Json parameter to REST module in suite configuration can be used to import only Json methods and part: Xml - only Xml methods.

See [Part] line under each method in https://codeception.com/docs/modules/REST

[ThomasLandauer]

This is ready for merging now.

Question: What's the reason for having those parts in this module? Is there any other module that conflicts with the entire REST module?