Create a redefining state in the registry

[hgrecco]

As pointed out in #1773, when redefining a unit, certain caches needs to be invalidated. The easy way is to do it after each definition but this is expensive if it is used many times. One proposal would be to create a defining state to the registry to make this more performant.

In pseudo code would look something like this:

class UnitRegistry:

    def define(self, ...):
        # actual code
        if not ureg._defining:
            self._build_cache()

    def defining():
        self._defining = True
        yield
        self._build_cache()
        self._defining = False

So single redefinitions can be added like now (but cache will be rebuild):
```python
ureg.define(...)

and when multiple are done

with ureg.defining():
    ureg.define(...)
    ureg.define(...)
    # and many more

The other proposal would be to detect if a redefinition has occur. But we have to be careful about all alias, prefixed units and stuff. I like this second option more, but we need to be sure to make it right.

[hgrecco]

    def load_definitions(self, ...):
        # actual code
        if not ureg._defining:
            self._build_cache()

[em1208]

@hgrecco The solution you proposed using defining looks fine to me but does it work also when using a new context and not just when adding a new definition?

[hgrecco]

Yes, the idea is that defining gets active when either load_defintions or define is called. And when it is then done, the cache is rebuilt.

[em1208]

Sounds good. How about when calling enable_contexts?

[em1208]

Hi @hgrecco, just checking, when do you plan to implement the proposal? Thanks!