Disperse velocities to ensure causality

[bsmithyman]

Need to modify MultiFreq to use dispersed real velocity when the complex velocity is non-zero. Will also require a base frequency; possibly only disperse velocities if baseFreq is set.

[bsmithyman]

@ShaunHadden I just added this issue to track the need for dispersing the velocities to ensure causality. This is one of the topics that came up in the meeting with Gerhard this morning. It should be semi-general for all of the methods multi-frequency methods, so we should probably implement it in MultiFreq. Just FYI for now.