WIP refactor to separate functions and add tests

[cassidymwagner]

This is a large refactor to improve the readability of the code and add substantial testing.

Summary of updates

• advection_velocity is now generate_structure_functions and can return advective and traditional structure functions for velocity and scalar data.
• Separated some functions from generate_structure_functions to create:
  • bin_data: bins the separation distances and structure function array in the case of uneven data
  • shift_array1d and shift_array2d: shift arrays (1D and 2D) by a given integer (set within generate_structure_functions based on number of grid points)
  • calculate_separation_distances: gets separation distances for uniform and latitude-longitude grids
  • calculate_structure_function: calculates the structure function for two points of data (called within generate_structure_functions, does not return the full array of structure functions for each separation distance). Supports advective and traditional structure functions for velocity and scalar data.
• calculate_velocity_advection and calculate_scalar_advection have been combined into calculate_advection and can therefore calculate either. Supports uniform and latitude-longitude grids.
• All functions have docstring
• All functions have tests. More can be added and are welcome

[cassidymwagner]

Going to have to update the tests since the new refactor returns data in a different format.

[cassidymwagner]

A brief demonstration that the results still match Pearson et al. 2021. (Fewer snapshots used.)