Add bout_v6_coordinates_upgrader script

[tomc271]

Add a python script that upgrades files to use the refactored Coordinates class.

[dschwoerer]

This seems to depend on boutproject/BOUT-dev#2873

Concerning boutproject/BOUT-dev#2873 (comment) - is the use of proxy classes still considered?
If so, it would be great if they could make it to v6 as well. Having tools to do the conversion is great, but if one wants to bisect, having libraries that are neither backwards nor forwards compatible is a huge pain.
I would really love if that could be done.

[ZedThree]

@dschwoerer It's related to those changes, but doesn't depend on them

Proxy classes would be nice, but they do also have significant downsides -- more difficult to debug, and tend to be fragile.

[dschwoerer]

@dschwoerer It's related to those changes, but doesn't depend on them

@ZedThree maybe we mean different things by "depend on the PR" - i meant that users would only want to use this conversion script, iff the PR is applied to their BOUT++ version. Otherwise that certainly would break the code?

I think we could make v5 forward compatible by:

• adding setters and getters
• Allowing to call Field3D's with returning the field itself (with raising a warning)

Proxy classes would be nice, but they do also have significant downsides -- more difficult to debug, and tend to be fragile.

I misunderstood / misremembered your comment, and thought that would be intended to be part of #2873 - but now I think that is not planned?