subbeamnod() should be renamed

[mpound]

Feature description
subbeamnod() method of GBTFITSLoad is the only calibration routine that doesn't start with get. Initially, we thought we would not be using getthis, getthat, and I named subbeamnod accordingly. However, we have used get so, we should change it to getsbnod or something like that.

Solution
Straightforward substitution but requires tests, notebooks, and documentation to change too.

[teuben]

I second this, but I could also argue to call it getnodsb() instead. This to favor those that use TAB-completion.