improving the distinguishing of the squitter length.

[hornig]

https://github.com/aerospaceresearch/PlaneSpotting/blob/5c44881bde6b35c86bfe6e27673ae064346fc8a4/main.py#L144-L151

currently the only way to distinguish between the 56bits and 112bits longh squitter messages is included here by a check of both by the checksum.
maybe there is a faster way to already detect which is it and only do the CRC check for one of them.

currently it takes a lot of time. it should be faster.