Add 2d vector field tab to the scatter plot viewer

[PennyQ]

As shown below, vx and vy are arrow vectors, the arrow dimensions (except for length) are measured in multiples of the square root of x square plus y square, ref to matplotlib_quiver definition.

[PennyQ]

The test failed because of astropy_table as below:

glue/core/data_exporters/tests/test_astropy_table.py::test_astropy_table[ascii.ipac] FAILED
glue/core/data_exporters/tests/test_astropy_table.py::test_astropy_table[ascii.latex] FAILED

[astrofrog]

@PennyQ - thanks for working on this! I wonder whether it would be good to have an option to select length and angle instead of vx/vy (I think both would be useful). Also can you add a checkbox to control whether to show the arrow heads?

[PennyQ]

@astrofrog The hide arrow and angle/length options are added within the new PR. As you can see below, please let me know anything else can be improved :)

[astrofrog]

I rebased this as I think it may fix the failing tests

[astrofrog]

@PennyQ - I fixed the issues, and this looks good, so merging! I'll tweak the UI of this and other things in the scatter plot viewer in a separate pull request.