RO-Crate version selector

[simleo]

Closes #230.
Closes #236.

>>> from rocrate.rocrate import ROCrate
>>> crate = ROCrate()
>>> crate.version
'1.2'
>>> crate = ROCrate(version="1.0")
>>> crate.version
'1.0'
>>> crate.metadata.version
'1.0'
>>> crate.metadata.id
'ro-crate-metadata.jsonld'
>>> crate = ROCrate("test/test-data/crate-1.1", version=1.2)
>>> crate.version
'1.1'

The context version is automatically set to the crate (metadata) version.