Make boost po opt-in, convert all test input files to getpot format

[dmcdougall]

Do not merge this until we have a release branch.

[codecov]

Codecov Report

Merging #457 into dev will increase coverage by 1.66%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev     #457      +/-   ##
==========================================
+ Coverage   69.45%   71.12%   +1.66%     
==========================================
  Files         170      169       -1     
  Lines       15135    14443     -692     
==========================================
- Hits        10512    10272     -240     
+ Misses       4623     4171     -452

Impacted Files	Coverage Δ
src/core/src/BaseInputOptionsParser.C	14.28% <0%> (-85.72%)	⬇️
src/basic/src/ScalarFunction.C	76.59% <0%> (-14.32%)	⬇️
src/misc/src/Miscellaneous.C	73.27% <0%> (-12.45%)	⬇️
src/stats/src/MLSamplingOptions.C	56.41% <0%> (-5.3%)	⬇️
src/core/src/OptimizerOptions.C	74.35% <0%> (-5.24%)	⬇️
src/stats/src/MLSamplingLevelOptions.C	89.55% <0%> (-3.58%)	⬇️
src/gp/src/GPMSAOptions.C	71.3% <0%> (-0.07%)	⬇️
src/core/src/BoostInputOptionsParser.C
...c/core/src/InfiniteDimensionalMCMCSamplerOptions.C	3.44% <0%> (+0.81%)	⬆️
src/core/src/Environment.C	68.97% <0%> (+1.75%)	⬆️
... and 5 more

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