fix more build warnings

[dksmiffs]

Continued work on #43.

[codecov]

Codecov Report

Attention: Patch coverage is 90.52925% with 34 lines in your changes missing coverage. Please review.

Project coverage is 49.68%. Comparing base (58d0bd8) to head (32e0063).

Files with missing lines	Patch %	Lines
src/KDataStream.cpp	0.00%	4 Missing ⚠️
src/PDU/Synthetic_Environment/Gridded_Data_PDU.cpp	0.00%	4 Missing ⚠️
src/DataTypes/EnvironmentRecord.cpp	0.00%	3 Missing ⚠️
src/PDU/Minefield/Minefield_Data_PDU.cpp	0.00%	3 Missing ⚠️
src/DataTypes/DeadReckoningParameter.cpp	71.42%	2 Missing ⚠️
src/DataTypes/IOEffect.cpp	0.00%	2 Missing ⚠️
src/DataTypes/LinearObjectAppearance.cpp	0.00%	2 Missing ⚠️
src/DataTypes/Mode5TransponderBasicData.cpp	0.00%	2 Missing ⚠️
src/DataTypes/PointObjectAppearance.cpp	0.00%	2 Missing ⚠️
src/DataTypes/VariableDatum.cpp	0.00%	2 Missing ⚠️
... and 7 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #71      +/-   ##
==========================================
+ Coverage   47.67%   49.68%   +2.00%     
==========================================
  Files         298      306       +8     
  Lines       26930    27128     +198     
  Branches     1054     1056       +2     
==========================================
+ Hits        12840    13479     +639     
+ Misses      14090    13649     -441     

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