reinterpret_cast should maybe be static_cast

[adamnovak]

Mac Clang is complaining we don't know how to cast right:

src/path_position_overlays.cpp:431:16: warning: 'reinterpret_cast' to class 'handlegraph::MutablePathDeletableHandleGraph *' from its virtual base 'handlegraph::PathHandleGraph *' behaves differently from 'static_cast' [-Wreinterpret-base-class]
        return reinterpret_cast<MutablePathDeletableHandleGraph*>(graph);
               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/path_position_overlays.cpp:431:16: note: use 'static_cast' to adjust the pointer correctly while downcasting
        return reinterpret_cast<MutablePathDeletableHandleGraph*>(graph);
               ^~~~~~~~~~~~~~~~
               static_cast

We could get into trouble with pointer offsets depending on how any given compiler is doing virtual base classes.

[jeizenga]

There's also the option of storing the pointer twice with different properties: once in the mutable subclass and once in the immutable parent class.