Allow graph_tool.draw.draw_hierarchy to be used without GTK installed

See merge request !29

Remove boost::any wrapper around std::shared_ptr

See merge request !28

Remove const qualifier from object that can be modified

See merge request !27

std::shared_ptr provides a limited amount of type erasure natively
based on void* and the fact that it stores the original type in the
deleter. As a result, if you know the original type, you can call
std::static_pointer_cast and recover a legitimate shared_ptr of the
right type, very efficiently.

It looks like the use of boost::any in this code is:

1) to hide the pointed-to type so a uniform container can be used
2) to represent "no pointer"

both of which can be handled natively by shared_ptr<void> with a small
performance gain from removing one indirection.

Mark PythonEdge as "final" per clang-tidy

See merge request count0/graph-tool!26

Modifying an object declared const is technically undefined behavior,
and compilers are entitled to do as they like with such code. The most
likely result - in the event of some future optimizer - would be
elimination of the code performing the modification, although it could
theoretically be anything.

Given that ConstantPropertyMap defines an assignment operator already,
simply changing the underlying data member to non-const (while making
it private) seems to achieve the same result as the current code while
avoiding UB.

Classes and methods marked final have some optimization possibilities
for the compiler, as some vtable lookups can be eliminated.

This fixes issue #662

This fixes issues #653 and #655

This fixes issue #658

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