cmake: fixed recognition of standard threading support for MSVC
fixed warning in isnan() on Windows
cleaned-up CoupledScanOrderIterator and related (added 'multi_fwd.hxx', moved CoupledHandle to 'multi_handle.hxx', added MultiHierarchicalIterator)
added missing 'template' keyword
changed unionFindWatersheds(Graph) to use local rather than global neighbor ID