Replace "NCP" with "server" in XSDs

[wrygiel]

With the appearance of EWP's ToRs API, EMREX ELMO format might become used outside of the EMREX project.

This means, that some developers might use the EMREX ELMO format without actually implementing EMREX. For those developers, references to "NCP" or "EMREX Client" in the XSD, might be a bit confusing.

It's worth considering adding some additional explanation for such developers, and possibly changing some EMREX-request-specific terms (like "NCP") with more generic ones (like "server"). "NCP implementer" is only a subset of all "server implementers" which may use EMREX ELMO format in the future.