Include error from eigen3

[mohd-akram]

Eigen3 does an #include <core/ngcore.hpp> which fails:

In file included from /opt/local/include/eigen3/../general/myadt.hpp:16:
In file included from /opt/local/include/eigen3/../general/../include/mydefs.hpp:14:
In file included from /opt/local/include/core/ngcore.hpp:4:
In file included from /opt/local/include/core/archive.hpp:23:
In file included from /opt/local/include/core/exception.hpp:9:
/opt/local/include/core/ngcore_api.hpp:4:10: fatal error: 'netgen_config.hpp' file not found
    4 | #include "netgen_config.hpp"
      |          ^~~~~~~~~~~~~~~~~~~
1 error generated.

The file netgen_config.hpp is not in the same directory as ngcore_api.hpp.

Presumably, either the include should be #include "../include/netgen_config.hpp" or the netgen_config.hpp file needs to be moved inside core.