ENH: Add "horizontal" option to `wavepwr()` function

[KCollins]

Description

Add a parameter option "horizontal" (in addition to Bx, By, Bz) that yields horizontal magnitude: the square root of the sum of squares of Bx and By.

Potential impact

Captures information about the Hall current.

Potential solution(s)

Alternate parameter input.

Alternatives

Might want to implement this in magdf() instead of wavepwr? (Should confirm)

Additional context

cf. Fukushima theorem 1970 (see first slides in Goddard seminar)
Bx = N/S, toroidal mode; By = E/W, poloidal mode

• Add to magfetch()
• Add to magdf()