Update NEXRAD Level2File for TDWR

[dopplershift]

NWS is planning to collect Level 2 data from TDWR's, using the a variant of the standard level 2 format. We need a few updates to handle this properly.

1. Remove assertion checking radial length against the offset in _decode_msg31; at best this should be a warning, but I'm pretty sure it should just be debug output if anything
2. Fix conditional decoding of versions of radial constant block. This should look at the encoded size of the block rather than using the volume version value.
3. Fix warning on size of Message 2 format; again, should probably use encoded message size as check.

This sample file should decode with no warnings by default.

A few other things I noticed:

1. We should remove all assertions from Level2File. None of the remaining ones look like conditions in which we will fail to walk through the messages in the file or do memory-exhausting tasks (due to bad sizes/offsets). These should just be log messages at the appropriate level
2. When reading the pointers to the data blocks, we should be looking at the number of blocks/size of the data block header to determine what to read, not just read in a fixed size. Based on the ICD, some pointers may not even exist. Should probably run through some files to see what's going on in practice.