Add SREM / SSEM and DSRC library JSON Schemas

[mathieu1fb]

What's new;

JSON Schemas:
Added SREM and SSEM schemas, alongside a DSRC schema regrouping common definitions for both SREM and SSEM (to be completed for future needs).

closes #184 and closes #183

---

What to do:

1. Read the new schemas and check them against ETSI repository docs for potential mistakes.
2. Evaluate and discuss the impact of the use of new DSRC "units" before merging.

[Hugues360]

In another issue on mobility messages ( #137) it was asked to create directories for each message. Perhaps you can put each message in a directory (and that will test relative link to dsrc?)
Nota: in the PR linked to the same issue, I have change the "type" field of the header in "message_type". If it is approved, it should be the case for this messages too (can be done also in a 2.0.O version?)

[ymorin-orange]

In another issue on mobility messages ( #137) it was asked to create directories for each message. Perhaps you can put each message in a directory (and that will test relative link to dsrc?)

Nota: in the PR linked to the same issue, I have change the "type" field of the header in "message_type". If it is approved, it should be the case for this messages too (can be done also in a 2.0.O version?)

I think we should decide now what we do, either one of:

• we merge the SREM/SSEM schemas first, so thus they should conform to the current practices and the PR doing the renaming and version bump should be rebased and would have to also update the SREM/SSEM schemans, or
• we merge the renaming and version bump first, so thus the SREM/SSEM schemas should conform to the new practices.

The second option sounds more appealing to me, but this means the new practices must be properly documented so as to simplify those new schemas.

[ymorin-orange]

I think we should decide now what we do, either one of:
* we merge the SREM/SSEM schemas first [...]
* we merge the renaming and version bump first [...]

We decided for the first option: merge SREM/SSEM schemas first.