math/arpack-ng: Update 3.5.0 -> 3.6.3

Port changes:

• Remove the slave port math/arpack-ng-mpich: the MPI option is incremental and only adds a library
• Permanently enable the MPI functionality: see above
• Shorten the CONFLICTS statement
• Install arpack.h which is forgotten by the project itself

Bump cad/elmerfem: dependency there changed to math/arpack-ng.