ENH: various dlLibraryTable improvements/refinements (#1737)
- libs() singleton method for global library handling - explicit handling of empty filename for dlLibraryTable open/close. Largely worked before, but now be more explicit about its behaviour. - add (key, dict) constructor and open() methods. More similarity to dimensionedType, Enum etc, and there is no ambiguity with the templated open(). - construct or open from initializer_list of names - optional verbosity when opening with auxiliary table, avoid duplicate messages or spurious messages for these. - basename and fullname methods (migrated from dynamicCode). - centralise low-level load/unload hooks - adjust close to also dlclose() aliased library names.