Global Quantity, Unit, and Measurement

[crusaderky]

This PR addresses several issues I found myself facing when using pint in my own projects.

• I found myself systematically needing to reference UnitRegistry, Quantity, and Unit in sphinx docstrings and typing annotations.
• When prototyping and writing snippet code, I got frequently annoyed by having to typing ureg = pint.UnitRegistry() every time, even when I was perfectly content with defaults_en.txt (as most novice users will).
• I found myself needing to access pint._APP_REGISTRY when creating a Quantity in a dask.distributed function.

Changes:

• Define global objects pint.Quantity, pint.Unit, and pint.Measurement. They can be used directly, e.g. pint.Quantity(1, "kg"). They use the global application registry.
• Fix bug where Measurement objects would be accidentally upcast to Quantity upon pickling
• All Exception classes are now accessible from the top-level module (also in intersphinx)
• New function pint.get_application_registry
• Documentation for using pint.set_application_registry in one's application
• intersphinx users can now reference :class:`pint.UnitRegistry` instead of the internal implementation module :class:`pint.registry.UnitRegistry`
• Fixed a wealth of sphinx errors (not all)
• Unit tests for pickling/unpickling custom units through the global registry

[crusaderky]

@hgrecco ready for review and merge

[crusaderky]

@hgrecco ping

[hgrecco]

I would like to test how this feels for a while.

[crusaderky]

@hgrecco ETA to either merge or tweak requests?

[crusaderky]

@hgrecco poke

[crusaderky]

@hgrecco if you don't have time to go through this yourself, could you please nominate deputies? It's been 23 days now...

[jthielen]

@hgrecco Any updates on the status of this PR? It would be great to see this make it in soon.

[hgrecco]

I have been playing with this for a while:

1. I like how it feels
2. I like the implementation

but, it seems to me that a changed is needed in the method/property SharedRegistryObject._Registry. This method is triggered when the _Registry attribute has not been set (i.e. when the Quantity was not created after a UnitRegistry is created). If I am reading everything correctly, there is a chance that the registry the Quantity referes first to a Registry and then to other if the application registry is changed by the user in between. This is not a good idea.

If that is the case, I would suggest that after calling SharedRegistryObject._Registry property once, the value is set as an attribute. I know that user can still change the _Registry attribute (It is Python after all) but it would require the user to write in an explicit manner.

[crusaderky]

@hgrecco good points. My latest commits address them.

[hgrecco]

bors r+

[hgrecco]

Thanks @crusaderky for all the work and congratulations!

[bors]

Build succeeded

• continuous-integration/travis-ci/push