Add Error Message that compiling backwards to 1.3 is not possible

[christian-rli]

I tried to parse and compile this file which first fails with the error described in #12 . After changing the null values to dates for testing purposes, I can parse without a problem.

Compiling the parsed metadata back to v1.4 works. Compiling to 1.3 yields this error:

    print(dialect1_3.compile(parsed))
  File ".../omi/env/lib/python3.7/site-packages/omi-0.0.2-py3.7.egg/omi/dialects/base/dialect.py", line 13, in compile
    c = self._compiler()
TypeError: 'NoneType' object is not callable

[MGlauer]

This is intended behavior, because there is no compiler for v1.3. The only output format is v1.4