Correlation: spec2d from saved synchronous sample

[hughesadam87]

Imagine I have a syncrhonous dataset from 2dShiege and I wanna open it as a Spec2d pyuvvis object. Now, I can't because _corr2d is hard-coded into Spec2d. _corr2d actually is just needed to access the original data, and its varunit/columns etc...

In this use case, the original data is lost. Therefore, need to refactor so that this can work when original data is missing. IE span_string and header return "Ivalid" or "??".

Perhaps the best use case is to force @classmethod from_corr2d because hard-baking init is bad practices, and then read_csv() overload for this use case.