Implement basic peripheral recovery

[iandanforth]

• [x] Add ability to disable central fatigue while maintaining peripheral
• [x] Disable central fatigue in a new muscle class that extends PotvinAndFuglevandMuscle
• [x] Implement basic recovery in the fibers model
• [x] Update examples to use new extended muscle class

[iandanforth]

5 Strategies for recovery

No recovery (Potvin 2017)

Linear recovery equal to fatigue rates

Linear recovery. Uses recovery rates from nominal fatigue rates scaled down by motor unit size

Asymptotic recovery using nominal fatigue rates

Asymptotic recovery using recovery rates from nominal fatigue rates scaled down by motor unit size