Hot: inspect fails for newliy written string variables

inspect fails for newliy written string variables

    sofa_test = sf.Sofa('GeneralTF')
    sofa_test.add_variable('TestString', 'test', 'string', 'S')
    sofa_test.inspect()

in this case sofa_test.inspect()fails, since 'test' does not have size attribute.
Not sure were to fix this, otherwise I would have opend a PR directly.

and this would work:

    sofa_test = sf.Sofa('GeneralTF')
    sofa_test.add_variable('TestString', 'test', 'string', 'S')
    sf.write_sofa(sofa_test, 'test.sofa')
    sofa_test = sf.read_sofa('test.sofa')
    sofa_test.inspect()

Start with the provided reproduction using Sofa.add_variable and Sofa.inspect, then compare the in-memory object with the object returned after write_sofa/read_sofa. Trace why inspect handles the round-tripped string variable but not the newly created one; done means inspect succeeds for the in-memory string variable without requiring serialization.