Refactored EK80 data XML parser and also removed the usage of dict_to_dict method

[praneethratna]

Fixes #468 by improving parsing for EK80 XML data and also removed the usage of dict_to_dict method, also Fixes #1184 and now 'PingSequence' XML datagram is parsed and stored in the output of open_raw.

CC @leewujung @emiliom

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.42553% with 9 lines in your changes missing coverage. Please review.

Project coverage is 67.85%. Comparing base (0bc534e) to head (ea66098).
Report is 87 commits behind head on dev.

Files with missing lines	Patch %	Lines
echopype/convert/utils/ek_raw_parsers.py	90.21%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1186      +/-   ##
==========================================
- Coverage   77.04%   67.85%   -9.20%     
==========================================
  Files          67       19      -48     
  Lines        5908     3080    -2828     
==========================================
- Hits         4552     2090    -2462     
+ Misses       1356      990     -366     

Flag	Coverage Δ
unittests	67.85% <90.42%> (-9.20%)	⬇️

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