Completed std::wstring support.

[hchen99]

Tested following for this ticket for both std::wstring and std::string:

• std::wstring _wstring, std::wstring[2] vec2_wstring, and std::wstring* ptr_wstring
  • In input file:
    • xxx._wstring = "Hello"
    • xxx.vec2_wstring = ["Hello", "World"]
    • xxx.ptr_wstring = trick.TMM_declare_var_1d("std::wstring", 5)
• checkpoint
• TV

[coveralls]

coverage: 56.928% (+0.2%) from 56.774% — wstring-support into master