Naming: VersionSpecifier and VersionRange

[Hritik14]

Range resonates with the concept of interval which can be said to be

an interval is a group of numbers that includes all numbers between the beginning and the end

In my understanding, a VersionRange object does not contain both the bounds for a range
https://github.com/nexB/univers/blob/63bd5aec16ec95b5b811ede638ac225f3ab1f6c6/src/univers/version_range.py#L25-L28

A VersionSpecifier does come with multiple VersionRanges and might work for upper-lower bounds but that would require to treat the ranges inside a VersionSpecifier with a mathematical intersection, which doesn't make a lot of sense right now.

For eg: An intersection of semver:1.2.3,>=2.0.0 doesn't quite make sense.
Their union, though, is useful but then we are left without any upper-lower bound range.

Please correct me if I am wrong. Also, it would greatly help if you could add documentation for the exposed classes (like VersionSpecifier, VersionRange etc)

[Hritik14]

We need to work on a canonical syntax for version specifiers. The following representations have been proposed:

>>> "semver:5,(>=1.0,<=2.0),6"
'semver:5,(>=1.0,<=2.0),6'

>>> "semver:5, >=1.0 AND <=2.0, 6"
'semver:5, >=1.0 AND <=2.0, 6'

>>> "semver:5, >=1.0 & <=2.0, 6"
'semver:5, >=1.0 & <=2.0, 6'

>>> "semver:5 || (>=1.0 && <=2.0) || 6"
'semver:5 || (>=1.0 && <=2.0) || 6'

>>> "semver:5 | (>=1.0 & <=2.0) | 6"
'semver:5 | (>=1.0 & <=2.0) | 6'

[Hritik14]

This has been taken care of in the vers spec implementation #12