simplify hiearchy for path graphs

[glennhickey]

I'm not a super huge fan of the MutablePathMutableHandleGraph. In practice this going to be one of our goto classes. Do we really need the Mutable specifier twice? Here's a proposal:

• Get rid of MutablePathMutableHandleGraph and MutablePathDeletableHandleGraph
• MutablePathHandleGraph now means the Graph and Path is mutable
• Add DeletablePathHandleGraph which likewise had deletable paths and graph

This makes the header files less modular, but as discussed in #16, we probably don't want to move forward with cross-products of features.

[glennhickey]

AHA, I finally used MutablePathHandleGraph with only one Mutable:

https://github.com/vgteam/vg/blob/master/src/augment.hpp#L61-L65