NDCube.squeeze method

[mihailbankov]

Creates the NDCube.squeeze method as described in the issue, similar to the numpy equivalent.
This method removes (specified) axis with length 1.
Testing for the method is also included.
Resolves #616

[DanRyanIrish]

Hi @mihailbankov. Thanks so much for this PR! I've suggested a slight refactor to your implementation that doesn't rely on for loops, which should make the code simpler and more efficient.

Also, I suggest that the an error is raised if a non-length-1 axes tries to be squeezed. This is consistent with np.squeeze which many users will already be familiar with. That change is included in my suggested refactor.

Finally, scalar NDCubes aren't supported because WCS's need non-scalar arrays for their transformations between pixel and world values to make sense. So I've also included an error which is raised if the user tries to squeeze all axes.

Let me know your thoughts. Thanks again!

[DanRyanIrish]

At first glance, the test coverage does seem a bit weird. I would suggest making the changes to the main code, which will also require some changes to the tests. If the coverage still looks weird, we can take a closer look then.

[mihailbankov]

I changed the line for isinstance where I put int; On the last line i changed arr to item; Aside from that I think there are no real changes to the code you proposed. The tests were also changed to test the new code.

[DanRyanIrish]

We also need to test whether .squeeze() only squeezes the requested axes, rather than all length-1 axes.

[nabobalis]

Thanks for the PR @mihailbankov