iDynTree compilation fails with IDYNTREE_USES_ASSIMP with vcpkg from assimp 2020.01

[traversaro]

In CI (i.e. #661) we are using assimp from vcpkg 2019.10, so probably there have been some regression there.
The error is:

2020-04-02T10:19:01.2237957Z D:\a\idjl-software\idjl-software\idyntree\src\solid-shapes\src\InertialParametersSolidShapesHelpers.cpp(22,10): fatal error C1083: Cannot open include file: 'assimp/Importer.hpp': No such file or directory [D:\a\idjl-software\idjl-software\idyntree\build\src\solid-shapes\idyntree-solid-shapes.vcxproj]

that is quite similar to the error that we get with assimp from homebrew, see #599 .

[traversaro]

Apparently the PR #661 was actually wrong, because even with Assimp >= 5.0 the assimp::assimp imported target was not linked. This should be fixed by #666 .