Investigate improvements to clean up complex indexing

[dopplershift]

There are a lot of places where we assemble list of slices and whatnot in order to do complicated index operations. This code got a lot uglier in the process of clearing warnings for numpy 1.15 by putting a lot of calls to tuple() everywhere. At some point we should look at this stuff and see if there are better ways:

• Better use of functions built into NumPy itself
• Implement some kind of object in MetPy to encapsulate these things
• Need to consider how this will play with XArray as we continue to move forward on that.