container/mts: Function to extract/compute frame period (1/fps) from MTS.

[Alan Noble]

It is proposed that the period be represented in MTS frequency units, i.e., 3600 for 25 fps, or 3000 for 30 fps, 1500 for 60 fps, etc.

[Alan Noble]

MTS frame period (1/fps) can be calculated by calling the new GetPTS() function repeatdedly.

Implemented in https://bitbucket.org/ausocean/av/commits/590bdc2c2d4c1c7fc1cd6bc0e1a131151c61f506