refactor: drop Boost.MPL dependency

[Becheler]

• Remove the Boost.MPL dependency, replacing its metafunctions with C++11-compatible equivalents:
  • STL stuff (std::conditional, std::integral_constant, static_assert)
  • void_t detection idiom with boost::void_t for the has_xxx traits.
• property_map_iterator.hpp was not exercised by any test: added property_map_iterator_test.cpp
• No public API break

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.80%. Comparing base (e50f160) to head (30f651f).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #38      +/-   ##
===========================================
+ Coverage    92.06%   92.80%   +0.74%     
===========================================
  Files            5        6       +1     
  Lines          126      139      +13     
  Branches        47       48       +1     
===========================================
+ Hits           116      129      +13     
  Misses           6        6              
  Partials         4        4              

Files with missing lines	Coverage Δ
...nclude/boost/property_map/compose_property_map.hpp	100.00% <ø> (ø)
...nclude/boost/property_map/dynamic_property_map.hpp	89.47% <100.00%> (ø)
...clude/boost/property_map/function_property_map.hpp	100.00% <ø> (ø)
include/boost/property_map/property_map.hpp	100.00% <ø> (ø)
...clude/boost/property_map/property_map_iterator.hpp	100.00% <ø> (ø)
...oost/property_map/transform_value_property_map.hpp	100.00% <ø> (ø)

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