New model

Implemented new functionality in Model:

• Implement __matmul__ (@) and change the behaviour of * to be consistent with the way this works in arrays.
• Support of sparse matrices and LinearOperator as values.
• Consistent support of scalar valued Models.
• Add keep to only keep track of certain symbolic coefficients.
• More options and more transparent initialization, allow string keys which are automatically symmpified by default.
• Several new utility functions, such as trace, reshape, etc.

Optimizations:

• Remove unnecessary deep copying, which was slow.
• Optimize the implementation of arithmetic operations to minimize number of loops and function calls.
• Fast initialization by making restructuring optional when initialized with a dict.
• Clean up the code of BlochModel to utilize improvements in Model.
• Update symmetry finder to work with sparse models.

Backward compatibility

• Changing the behaviour of * breaks backward compatibility, but aside from a few uses in the qsymm code base (fixed), we do not use it in the examples and not aware of anyone using this functionality.
• Deprecate initializing empty Model without providing shape and dtype.
• Stop supporting a sequence of ints as momenta
• Certain other ways to initialize Model don't work anymore, some tests were changed accordingly. This again is unlikely to break any existing code.
• Some tests in kwant.qsymm break because of changed internal format.