hgrecco / pint / 1413

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Merge #938

938: Use contexts to redefine units r=hgrecco a=crusaderky

- [x] Closes #853 - Context.define for currency conversions
- [x] Closes #932 - Disable all throwaway contexts
- [x] Contexts that only implement transforms between dimensionalities (that is, all contexts until now) no longer require their own cache and therefore are enabled in microseconds even on the first time; contexts that implement redefinitions require a *much* smaller cache than before which is very quickly built on demand.
- [x] NaN is now parsed as a float

- [x] Executed ``black -t py36 . && isort -rc . && flake8`` with no errors
- [x] The change is fully covered by automated unit tests
- [x] Documented in docs/ as appropriate
- [x] Added an entry to the CHANGES file


# Demo
```python
import pint

ureg = pint.UnitRegistry()
ureg.load_definitions("""
USD = [currency]
EUR = NaN USD
GBP = NaN USD

@context currencies_20191218
    EUR = 1.11 USD
    GBP = 1.18 EUR
@end

@context dungeons_and_dragons = dnd
    # Simplified conversion rates for the
    # non-English versions of the D&D manuals
    pound = 0.5 kg
    gallon = 4 l
    foot = 1.5/5 m
@end
""".splitlines())

>>> ureg.GBP.dimensionality
<UnitsContainer({'[currency]': 1.0})>
>>> ureg.Quantity("10 GBP")
10 GBP
>>> ureg.Quantity("10 GBP").to("USD")
nan USD
>>> ureg.Quantity("10 GBP").to("USD", "currencies_20191218")
13.098 USD
>>> ureg.Quantity("10 ft").to("m")
3.0479999999999996 meter
>>> ureg.Quantity("10 ft").to("m", "dnd")
3.0 meter
```


Co-authored-by: Guido Imperiale <crusaderky@gmail.com>

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