Cythonize Timestep for greater performance

[hmacdope]

Is your feature request related to a problem?

Belatedly realised that I forgot to open an issue.

For greater performance of the core Timestep object and its various roles in holding coordinate, box and reader data, Timestep should be made a Cython Extension type.

Describe the solution you'd like

• The Timestep object should be made a Cython extension type and all attributes that can be converted to a compiled representation should be.
• No change to the API behaviour permitted
• NumPy operations should use the NumPy C API wherever possible
• Outdated attributes and tests should be removed or changed appropriately
• Readers that subclass Timestep (INPCRD) should no longer use their own subclass.
• Readers that use the _frame attribute should now hold it in the ts.data dictionary rather than as an optional attribute
• All references to the old timestep in the docs should be updated
• A single surface where you can cimport the cython layer of core should be constructed.

Feedback welcome on PR #3683 !