Add in the initial support for DPD conversions

[mborland]

Closes: #735

[codecov]

Codecov Report

Attention: Patch coverage is 99.14894% with 2 lines in your changes missing coverage. Please review.

Project coverage is 99.2%. Comparing base (6725d33) to head (db3e924).
Report is 22 commits behind head on develop.

Files with missing lines	Patch %	Lines
include/boost/decimal/dpd_conversion.hpp	99.1%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #738     +/-   ##
=========================================
- Coverage     99.2%   99.2%   -0.0%     
=========================================
  Files          224     226      +2     
  Lines        16400   16635    +235     
  Branches      1779    1792     +13     
=========================================
+ Hits         16258   16490    +232     
- Misses         142     145      +3     

Files with missing lines	Coverage Δ
include/boost/decimal/decimal32.hpp	98.8% <ø> (ø)
include/boost/decimal/decimal32_fast.hpp	96.8% <ø> (ø)
test/test_dpd_conversions.cpp	100.0% <100.0%> (ø)
include/boost/decimal/dpd_conversion.hpp	99.1% <99.1%> (ø)

... and 1 file with indirect coverage changes

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