Make float conversions explicit again

[mborland]

Adds configuration macro BOOST_DECIMAL_ALLOW_IMPLICIT_CONVERSIONS if the user wants it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.7%. Comparing base (e109d0e) to head (f749270).

Additional details and impacted files

@@           Coverage Diff           @@
##           develop    #506   +/-   ##
=======================================
  Coverage     98.7%   98.7%           
=======================================
  Files          180     180           
  Lines        11457   11457           
=======================================
  Hits         11307   11307           
  Misses         150     150           

Files	Coverage Δ
include/boost/decimal/decimal128.hpp	97.4% <ø> (ø)
include/boost/decimal/decimal32.hpp	99.7% <ø> (ø)
include/boost/decimal/decimal64.hpp	98.9% <ø> (ø)
test/github_issue_426.cpp	100.0% <ø> (ø)
test/test_assoc_legendre.cpp	100.0% <100.0%> (ø)
test/test_legendre.cpp	100.0% <100.0%> (ø)

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