This fixes a problem which occurs when copying external property maps
when using undirected graphs.

Remove unused imports

See merge request count0/graph-tool!25

The version of nested_for_each() without boost::any arguments must be
called for every type, not only the first. This means the wrapped action
must return false, otherwise the compiler will omit the remaining calls.

This fixes issue #651

Remove exception throwing from hot path in dispatch

See merge request count0/graph-tool!24

Eliminate fail_cast and stop_iteration exceptions, replacing their
use with bool success codes and fold expressions.

Fix deprecation warnings

See merge request count0/graph-tool!23

This fixes issue #642

This fixes issue #619

This fixes issue #641