Modify for double support

Modify plotting function to plot the sum of forces and moments from both
feet in the case of double support.
Add new files for double support calculation

Add `caldyn_bothleg.m` to perform the dynamics calculation for double
support configuration.
Add `fmdistribute.m` to compute the distribution of forces and moments
according to a ratio.
Add `test_caldyn_bothleg.m` to test double support dynamics calculation.
Add `uppercom.m` to compute the center of gravity of the upper body.
Add `wtdistribute.m` to compute the distribution (ratio) of weight
between the left and right legs.
Add sensor data

Add force plate data - `*.csv` files.
Add motion capture data - `*.drf` files.