Bugfixes and improvements

• Closes #54

  • for some fields like speed, lat or lon their types were not properly
    enforced. This led to typing errors and wrong values, e.g. 0 instead
    of 0.0.

• Closes #59

  • when encoding messages the underlying method int_to_bin incorrectly
    assumed that all values were signed. This was wrong and could lead to
    invalid messages, when encoding.

• Closes #58

  • A missing checksum caused a ValueError, which is now ignored