Set gridline locators for all four axes of the plot

[icetianli]

Description

Cartopy sometimes produces maps with mixed latitude and longitude gridline labels on the same side of the plot, like this one mentioned in #1560:

The right axis has an unexpected longitude label 16 E. This looks a bit messy, especially in terms of producing maps of publication quality.

One solution is the custom function written by @friedrichknuth in #1560, it solves the issue of mixed lat/lon labels by modifying the gridline locators of each axis. However, this would require writing extra function everytime plotting the figure.

I was wondering if it is possible for cartopy to implement additional gridline locator parameters for all four axes of the plot, such as x_top_locator, x_bottom_locator, y_left_locator, y_right_locator, instead of just xlocator and ylocator in the current version. This will allow users to specify locators where they would like to label the gridline for any axis.

[greglucas]

There is some discussion in #1537 around how to draw the combinations x/y + sides. Feel free to chime in there with your thoughts and ideas.

[QuLogic]

Closed by #1537