pick_measure() needs to be more flexible

[gadenbuie]

I wrote pick_measure() assuming that columns in the cell segmentation data are named like

Entire Cell Opal 520 Mean

but I think that it's possible for these names to change based on settings in inForm because the phenoptr documentation shows

Entire Cell PDL1 (Opal 520) Mean

as a valid column name.

pick_measure() should be more like a tidyselect helper. I think I can use quotation inside gather_intensity() so that pick_measure() can get access to column names in the csd and from there I can use basic regex to pick the right name.