Add LoWPAN enhancements

[akestoridis]

• Create an mle layer
• Improve the dissection of secured MAC Data packets
• Improve the dissection of secured MAC Beacon packets
• Enable the dissection of Thread beacons
• Improve the dissection of secured MAC Command packets
• Improve the dissection of 6LoWPAN Mesh header fields
• Fix bug in the extraction of the source address of 6LoWPAN packets
• Improve the dissection of 6LoWPAN IPHC header fields
• Fix bug in the 6LoWPAN dispatcher
• Add unit tests

[codecov[bot]]

Codecov Report

Merging #3556 (14b4038) into master (e0c96c5) will decrease coverage by 0.01%. The diff coverage is 79.64%.

:exclamation: Current head 14b4038 differs from pull request most recent head 123db03. Consider uploading reports for the commit 123db03 to get more accurate results

@@            Coverage Diff             @@
##           master    #3556      +/-   ##
==========================================
- Coverage   86.18%   86.17%   -0.02%     
==========================================
  Files         284      285       +1     
  Lines       64874    65024     +150     
==========================================
+ Hits        55914    56032     +118     
- Misses       8960     8992      +32     

Impacted Files	Coverage Δ
scapy/config.py	81.16% <ø> (ø)
scapy/layers/dot15d4.py	78.80% <77.58%> (-0.77%)	:arrow_down:
scapy/layers/mle.py	80.64% <80.64%> (ø)
scapy/layers/sixlowpan.py	86.06% <89.47%> (+0.62%)	:arrow_up:
scapy/layers/zigbee.py	79.77% <100.00%> (-0.08%)	:arrow_down:
scapy/arch/windows/__init__.py	67.73% <0.00%> (-0.57%)	:arrow_down:
scapy/layers/ntp.py	84.55% <0.00%> (-0.28%)	:arrow_down:
scapy/contrib/isotp/isotp_soft_socket.py	83.23% <0.00%> (-0.20%)	:arrow_down:
scapy/fields.py	91.13% <0.00%> (-0.06%)	:arrow_down:
... and 1 more

[akestoridis]

Fixed compatibility with Python 2.7, added more unit tests, squashed them into one commit, and rebased against master.

[gpotter2]

Thanks for the PR, I have a few comments to start with. Very sorry for the review delay

[akestoridis]

Thank you for reviewing the PR. Unfortunately, I am unable to make further changes. However, you are welcome to refactor the code in a follow-up commit. The unit tests of this PR should pass even after the suggested changes.