Consolidate and organize utility functions

[braingram]

As brought up during #270 in comment #270 (comment) in reference to that PR, existing "utilites", and code added in #267 the expansion of common code in stcal is resulting in duplication (and revealing existing duplication of code).

So far the work has largely focused on moving code related to individual steps to stcal and it seems worthwhile to consider:

• how these utilities could be organized
• what code can be consolidated
• what code might find a better "home" in other shared libraries (gwcs, tweakwcs, etc)