Add sketch of new Tensor interface

[dennisYatunin]

This PR refactors the Geometry module to use a much simpler interface, so that all vector/tensor operations can expressed using standard math operations instead of custom API functions. This will allow us to remove a large amount of duplicate code, reduce compilation latency, and speed up GPU runs by optimizing the geometry data passed to each kernel.

• Code follows the style guidelines OR N/A.
• Unit tests are included OR N/A.
• Code is exercised in an integration test OR N/A.
• Documentation has been added/updated OR N/A.

[dennisYatunin]

This is looking pretty great! My main comments are that we should avoid allowing tensor adjoints to be specified in two different ways, and that a lot of the code remaining in conversions.jl can be simplified or eliminated. Also, it would be helpful to add detailed docstrings for Basis, Metric, Tensor, and TensorWithAnyBasis, with some examples that indicate how reshape works and when it gets called.