Fixes to Map++ C API

[sanguinariojoe]

Feature or improvement description

I am making a more consistent C API:

• Not exposing the internal data structures to the final user
• Importing the required headers on mapapi.h
• Enclosing the mapapi.h content on a extern C (for C++ only)

Related issue, if one exists
None

Impacted areas of the software
Map++

Additional supporting information
Not much else to add

Test results, if applicable

• Update the documentation version in docs/conf.py
• Update the versions in docs/source/user/api_change.rst
• Verify readthedocs builds correctly
• Create a tag in OpenFAST
• Create a merge commit in r-test and add a corresponding tag
• Compile executables for Windows builds
  • FAST_SFunc.mexw64
  • OpenFAST-Simulink_x64.dll
  • openfast_x64.exe
  • DISCON.dll