Expose value and unit in python.

[volubepossum]

As of now the direct attributes are not exposed at all in python, so the only way of getting them is through parsing the string representation, which needless to say is very tedious, especially for the units.

I ran into the issue while trying to store, read, then use the dimensioned values for further computation. An example use-case of this feature could be like this:

from si_units import *
import json

foo = 3 * METER
bar = json.dumps({"value": foo.value, "unit": foo.unit})
baz = json.loads(bar)
qux = SIObject(baz["value"], baz["unit"])
print(qux)

[prehner]

Hi, thank you for the suggestion! Internally, we use the __getnewargs__ function for these kind of use cases. However, I agree that having simple getters would be useful for cases like serialization in JSON. @g-bauer what do you think?

Just one small comment on the implementation: PyO3 provides descriptors that you can use to generate getters:

#[pyo3(get)]
value: Py<PyAny>,

For the unit, this does not work, but you can replace the getter for the value this way.

[g-bauer]

I think that's a useful addition! Looks good to me.