Warn of incompatible schema versions between client and server executors

[nokome]

If a Client (e.g. StdioClient in this repo) and Server (e.g. StdioServer in pyla) are using different major versions of Stencila Schema there are likely to be problems.

One approach to this is that servers publish the schema version as part of the manifest() and clients request that manifest and check the schema version against their own on initial connection. This should be fairly straight forward to implement