Strategy for versioning

[havogt]

Currently we pretend to be backwards compatible, i.e. a new serialbox version will read an archive from an earlier version. (This was changed recently, before it was a requirement to have the same version for writing and reading.)

Probably the patch version should be backwards compatible i.e. 2.1.x
but minor version should be incompatible 2.x.0.

[havogt]

I think we should introduce a separate version for archives, i.e. we only increase the archive version number if something changed in the archive. Then again the same question as above for this version number.
See also #151

[clementval]

I faced the problem again today! This is really crazy to check even patch number for archive compatibility.

Serialbox: ERROR: error while parsing "./ser_data/MetaData-current_jsb_ser_rank0.json": serialbox version of MetaData (2.4.3) does not match the version of the library (2.4.2)