This version formalizes the inclusion of new features introduced from 0.7a1
to 0.7b2. An (incomplete) list of features include:
- A redesigned active learning module (`graphdot.model.active_learning`).
- The PBR graph reordering algorithm for graph kernel acceleration
  (`graphdot.graph.reorder.pbr`).
- LOOCV predictions using the low-rank approximate GPR.
- Significant improvement to the robustness of the training methods of GPR
  and Low-rank GPR models.
- Allow kernel/microkernel hyperparameters to be declared as 'fixed' via the
  `*_bounds` arguments.
- Added a `DotProduct` microkernel for vector-valued node and edge features.
- Added a `.normalized` attribute to all elementary and composite microkernels.
- Graph representation string can now be directly deserialized using `eval`.
- New atomic adjacency options such as alternative bell-shaped compact
  adjacency functions (`compactbell[a,b]`), and new length scale choices using covalent radiu etc.
- Perform value range check for the node and edge kernels during graph
  kernel creation.
- Added a `to_networkx()` method to `graphdot.Graph`.
- Enhanced the readability of the string representations of kernel
  hyperparameters using an indented print layout.
- Various performance and bug fixes.