Add `__len__`, `size`, and `shape`

[prisae]

Following up on a discussion on Slack: we should add __len__, size, and shape attributes. For a TensorMesh, e.g., shape is the same as vnC, and size is the same as nC. However, while shape and size is familiar to anyone working with numpy, vnC and nC are not.

[prisae]

This was implemented in #227