Added a helper function for testing equality/closeness of longitudes

[ayshih]

Longitude testing is a little tricky because of the wrapping. This PR creates a new function to use in tests to check for the equality/closeness of longitudes.

Should fix #3801

[pep8speaks]

Hello @ayshih! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2020-02-18 02:56:42 UTC

[dstansby]

I think this should probably go upstream to astropy, given it's non SunPy specific.

[wtbarnes]

I don't really know what the milestone should be here?

[sunpy-backport]

The backport to 1.0 failed:

Commits ["d111b51cc2ca5f74a18b099840dbb698dabfcdc0"] could not be cherry-picked on top of 1.0

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub.
git fetch
# Create new working tree.
git worktree add .worktrees/backport 1.0
# Navigate to the new directory.
cd .worktrees/backport
# Cherry-pick all the commits of this pull request and resolve the likely conflicts.
git cherry-pick d111b51cc2ca5f74a18b099840dbb698dabfcdc0
# Create a new branch with these backported commits.
git checkout -b backport-3804-to-1.0
# Push it to GitHub.
git push --set-upstream origin backport-3804-to-1.0
# Go back to the original working tree.
cd ../..
# Delete the working tree.
git worktree remove .worktrees/backport

Then, create a pull request where the base branch is 1.0 and the compare/head branch is backport-3804-to-1.0.

[sunpy-backport]

The backport to 1.1 failed:

Commits ["d111b51cc2ca5f74a18b099840dbb698dabfcdc0"] could not be cherry-picked on top of 1.1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub.
git fetch
# Create new working tree.
git worktree add .worktrees/backport 1.1
# Navigate to the new directory.
cd .worktrees/backport
# Cherry-pick all the commits of this pull request and resolve the likely conflicts.
git cherry-pick d111b51cc2ca5f74a18b099840dbb698dabfcdc0
# Create a new branch with these backported commits.
git checkout -b backport-3804-to-1.1
# Push it to GitHub.
git push --set-upstream origin backport-3804-to-1.1
# Go back to the original working tree.
cd ../..
# Delete the working tree.
git worktree remove .worktrees/backport

Then, create a pull request where the base branch is 1.1 and the compare/head branch is backport-3804-to-1.1.