New traditional structure functions

[cassidymwagner]

New traditional structure functions: "LL", "LLL", "LTT", "LSS". Now user can specify one of these strings or a list of a combination of these strings to generate these particular structure functions.

• LL: longitudinal 2nd order SF
• LLL: longitudinal 3rd order SF
• LTT: longitudinal-transverse-transverse 3rd order SF
• LSS: longitudinal-scalar-scalar 3rd order SF

Misc:

• Support periodic boundary conditions in x or y or both instead of none or both.
• Bug fix in calculate_advection for nonuniform gridding.

[cassidymwagner]

Need to add tests and rework old ones that are now failing.

[cassidymwagner]

Old tests now work, need to add test for LTT structure function and then everything should be good to go.