merged 3 implementations of numberList(QComboBox*, float, ...)

Summary: merged 3 implementations of numberList(QComboBox*, float, ...)

Test Plan: run diana GUI, check field, vcross, obs dialog

Reviewers: #diana, lisbethb

Reviewed By: #diana, lisbethb

Subscribers: lisbethb

Differential Revision: https://phab.met.no/D62