Fix tf2_eigen for MSVC

[Ace314159]

Trying to use any of the convert functions results in an ambiguous call compile error on MSVC. Here is an example:

geometry_msgs::msg::Quaternion quat;
tf2::convert(Eigen::Quaterniond(), quat);

This is because the Eigen namespace contains some implementations of toMsg and fromMsg. These implementations seem to be required on other compilers like Clang and GCC because of something called two-phase name lookup, which is explained in a comment in the code as well as this Microsoft blog post.

However, as stated in that blog post, two-phase name look up is not enabled by default on MSVC because it was recently added. In order to enable it, the compiler flag /permissive- needs to be used. With that flag, the existing code does compile, but having to use a compiler flag just to use this library doesn't sound ideal.

Instead, I think it would be better to just not have those implementations in the Eigen namespace for MSVC as I have done.

[traversaro]

As this modification is done in a header file, it may be eventually included in a compilation unit that is compiled with /permissive- . Do you have any idea if there is a macro to detect in the preprocessor if /permissive- is used, so the header will work both for MSVC without /permissive- and with /permissive-?

[Ace314159]

@traversaro Good catch! I hadn't considered that. Regarding a macro, all I could find was a request to add a macro here, but it looks they haven't actually added it.

Perhaps, the solution might be to just force everyone using MSVC to use the /permissive- flag. Do you have any other ideas?