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Merge #1150

1150: Add option to disable sorting of formatted unit components r=hgrecco a=lukelbd

- [ ] Closes # (insert issue number) (*n/a*)
- [x] Executed ``black -t py36 . && isort -rc . && flake8`` with no errors
- [ ] The change is fully covered by automated unit tests (*do I need to address this?*)
- [x] Documented in docs/ as appropriate
- [x] Added an entry to the CHANGES file

This is a very simple PR that adds a `sort` keyword argument to `pint.formatting.formatter`. The `sort` keyword can be set to `False` to optionally disable alphabetical sorting of the component units. This can be useful if your units are conventionally placed in a particular order, or conceptually easier to understand in a particular order.

Also, since preserving the dictionary insertion order is [an explicit python 3.6+ feature](https://docs.python.org/3/whatsnew/3.6.html#new-dict-implementation), the `unit: exponent` pairs in the `UnitsContainer` dictionaries can be reliably expected to preserve their original order, making this feature practically useful for formatting existing `Units` objects.

Here's a quick example with the conventional [specific heat capacity](https://en.wikipedia.org/wiki/Specific_heat_capacity) units:

```python

In  [1]: import pint
    ...: ureg = pint.UnitRegistry()
    ...: units = ureg.parse_units('J kg^-1 K^-1') 

In [2]: units.format_babel('C~', sort=True, as_ratio=False, product_fmt=' ')
Out[2]: 'J K**-1 kg**-1'

In [3]: units.format_babel('C~', sort=False, as_ratio=False, product_fmt=' ')
Out[3]: 'J kg**-1 K**-1'
```

Co-authored-by: Luke Davis <lukelbd@gmail.com>

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