Do the overall change and check on MVBS_ds

[ldr426]

We can do the overall change from MVBS_ds to just ds.

To ensure people put in the right type of dataset, we should add a input checking method that is called in Echogram.__init__ to check for the following:

• existence of the Sv data variable
• Sv data variable should have dimensions ("channel", "ping_time", "echo_range")

We should also allow depth in the last dimension, but we can do that later -- I'll create an issue so that we do not forget.

Originally posted by @leewujung in #93 (comment)