Use throttle factor for thrust in frequency calculation

[taesungh]

Resolves #33.

Changes

• Convert thrust equation to use proportion of peak
  • To allow variable thrust output, use a throttle factor to calculate the slip for the desired proportion of the peak thrust
  • The shape of the thrust profile is independent of amplitude
• Update thrust constant values
  • Update air gap value
  • Use relative permeability to avoid multiplying and dividing by pi

Testing

Display the results of computing calculate_frequency for various inputs

(0, 0) -> 0
(0, 0.5) -> 2.67284
(0, 1) -> 9.97517