Need smarter volume calculations at the Movement level

[scottt2]

Performances can have different units within the same Movement. Right now, we fold all Performances up by multiplying the successful reps with the load to produce the total volume for the Movement. We then just slap whatever the assumed Movement unit is on that value.

Example

# unit: lbs

squat:
  100 10r
  100 10r
    # unit: kgs

This would produce a movement.volume of 2000 lbs but it should be 1000lbs + 1000kgs or roughly 3205lbs.

Perhaps we need a static table of conversions for common units that we can then coerce the specified session or movement unit from disparate performance units.