Add correct comparison support for Scalar

[arthursoprana]

This PR attempts to fix <= and >= comparisons with Scalar

Fix #26

[codecov]

Codecov Report

Merging #31 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #31      +/-   ##
==========================================
+ Coverage   97.47%   97.55%   +0.08%     
==========================================
  Files          53       53              
  Lines        9270     9339      +69     
==========================================
+ Hits         9036     9111      +75     
+ Misses        234      228       -6

[nicoddemus]

Please add a CHANGELOG entry

[prusse-martin]

Due to floating point rounding self._value == other.GetValue(self.unit) this could lead to situations where (a == b) != (b == a).
I think this should agree on the unit used for comparison. Maybe the one composed by composing quantities base units.

[arthursoprana]

I've refactored to do the comparison always using DefaultUnit:

    def __eq__(self, other):
        return (
            type(self) is type(other)
            and self._quantity._category == other._quantity._category
            and self._GetValueInDefaultUnit() == other._GetValueInDefaultUnit()
        )

    def _GetValueInDefaultUnit(self):
        try:
            return self.GetValue(
                self._unit_database.GetDefaultUnit(self._quantity._category)
            )
        except InvalidQuantityTypeError:
            return self._value

[prusse-martin]

Scalars with the same category could have different units...
I don't think returning the raw value is a good option in InvalidQuantityTypeError.

Maybe return (self._value, self._quantity._unit)? If you do something like this I recommend a docstring stating that.

I just noted that this is also used on almost equal. Sorry.

[arthursoprana]

hmm, ok. I've took you suggestion, but with a slightly modification, take a look.

[arthursoprana]

There are codebases that rely on the current comparison behavior. Decided not to merge it at the moment.