Progress on 3D

[tim-griesbach]

Progress and comments on 3D translations. In particular,

• thoughts and remarks in fclaw2d_domain.h and fclaw{2,3}d_global.h by @cburstedde,
• add declarations of fclaw{2,3}d_set_global_context and fclaw{2,3}d_clear_global_context by @cburstedde,
• translation and adjustment to 3D of the overlap exchange code by @hannesbrandt ,
• move fclaw2d_domain_is_meta and add fclaw2d_domain_init_meta to forestclaw{2,3}d.h by @hannesbrandt
• synchronize code and documentation of domain_search_points by @tim-griesbach and
• harmonize 2D and 3D documentation of domain_search_points by @tim-griesbach.