MAVExplorer: Fix stats command for DF text and TLOG logs

[shancock884]

When looking at something else, I noticed that when using the 'stats' command in MAVExplorer with either DF text logs or TLOGs it raises an Exception for different reasons.

• DF Text logs: KeyError: 128
• TLOGs: ERROR in command []: 'mavmmaplog' object has no attribute 'filehandle'

It looks like TLOGs were not supported, but should at least have given a nicer error message. But while here, I have added the support for them.

Summary of changes needed:
For DF text logs:

• message name rather than message id is used to index the counts dict, and it is unset for messages that are never seen.
• needs: DFReader_text: some meta-data dicts are indexed by bytes instead of str pymavlink#906

For TLOG:

• As there isn't a formats attribute on the mavmmaplog class (as used for Tlogs), I am counting message instances rather than size in bytes. Output formatting is adjusted to take account of the longest name.
• The 'filehandle' attribute is named 'f' instead in mavmmaplog class (as used for Tlogs), so updated MPDataLogChildTask class of multiproc_util.py to handle either, using 'hasattr'.

The new output for TLogs looks something like this (abbreviated):

Total number of messages: 96576
REMOTE_LOG_BLOCK_STATUS    0.00%
AUTOPILOT_VERSION          0.00%
...
VIBRATION                  3.17%
MISSION_CURRENT            3.17%
HEARTBEAT                  3.50%
NAMED_VALUE_FLOAT          6.25%

@OTHER 100.00%

I've tested files of all three types (bin, log, tlog), and checked stats command works in each case.

[peterbarker]

Would be nice to get tlog usages in bytes - useful for fixing link bandwidth issues.

We might just be able to do a len on the byte-content of the message which we stash away in the Python ovject somewhere, I think. OTOH, it might be zero-pad-extended or it might not have timestamps, so delving reuqured!

Anyway, thanks, this has bugged me for a while, merging it!