Optionally only install the group holding the incoming dependencies

[rjyunis]

This pull request adds the --only flag to poetry add. The rationale is to counteract the following situation:

poetry install --without=somegroup
poetry add new-dependency
# Poetry now installs new-dependency as well as dependencies in somegroup

By invoking --only the group of the incoming dependencies is passed to the install command via self.installer.only_groups(group).

This is the best solution/simplicity ratio I have found to mimic the current workaround

poetry add new-dependency --lock
poetry install --without=somegroup

Tests incoming.
Doc updates incoming.

Pull Request Check List

Resolves: #issue-number-here

• Added tests for changed code.
• Updated documentation for changed code.