Issue 2644: Rename constants to context

[PeterWolfram]

As described in issue #2644

1. renamed FieldSet.add_constant -> FieldSet.add_context
2. renamed FieldSet.constants -> FieldSet.context
3. added setattr to raise an error if trying to attribute access a variable in FieldSet.context to prevent silent unexpected behavior

Additionally deleted empty "_core/constants.py" file

[erikvansebille]

Looks good; just a few small comments below!

[erikvansebille]

This is a v3 file; so shouldn't be changed. Can you revert this change? The same is true for the other files in docs/user_guide/examples_v3

[VeckoTheGecko]

git tip for Peter: you can quickly revert by doing git checkout main -- docs/user_guid/examples_v3. After the -- specifies files, which is basically like "copy the files from the main branch"

[erikvansebille]

what changed here? Whitespace only?

[VeckoTheGecko]

Removed file :)

[erikvansebille]

This is an old file from tests-v3; these don't need to be changed

[VeckoTheGecko]

Also, small note - would you mind following our PR template? It nice to keep things consistent, and also the Closes #xxx syntax means that the upstream issue gets auto-closed when this is merged