Add eval script for body metrics

[brentyi]

This isn't the exact script we used for the paper metrics, but should have the details that matter matched. Below are some metrics from this script when our released checkpoint is passed in.

For --subseq-len 128:

     mpjpe 118.340 +/- 1.350             (in paper: 119.7 +/- 1.3)
     pampjpe 100.026 +/- 1.349           (in paper: 101.1 +/- 1.3)
     T_head 0.006 +/- 0.000              (in paper: 0.0062 +/- 0.0001)
     foot_contact (GND) 1.000 +/- 0.000  (in paper: 1.0 +/- 0.0)
     foot_skate 0.417 +/- 0.017          (not reported in paper)

For --subseq-len 32:

     mpjpe 129.193 +/- 1.108             (in paper: 129.8 +/- 1.1)
     pampjpe 109.489 +/- 1.147           (in paper: 109.8 +/- 1.1)
     T_head 0.006 +/- 0.000              (in paper: 0.0064 +/- 0.0001)
     foot_contact (GND) 0.985 +/- 0.003  (in paper: 0.98 +/- 0.00)
     foot_skate 0.185 +/- 0.005          (not reported in paper)

related: #7