Add a compute HFR option, and separate HFR computation from Mark Stars.

Currently, the only way to get the fits module to compute the HFR of a captured image, one has to select the fitsviewer view->mark_stars option, which will then (a) compute the image's overall HFR (for this and future captures), (b) display the HFR on the status line, and (c) render circles around the stars used in the HFR computation.

This MR separates the two concepts (HFR computation and rendering). With a checkbox/option set, one can request that the HFR be computed for each (FITS_NORMAL) capture. The (circled stars) rendering may or may not happen, depending on the old view->mark-stars selection. If the HFR is computed but mark-stars is not set, then the HFR is still displayed on the fitsviewer's status bar, as before. It may also be written to the log, if the correct logging checkboxes are checked. Nothing else would be done with the HFR (as is currently the case).

I find these HFR values useful in evaluating an imaging session, and I have had a couple of requests for simplifying the current awkward workaround for getting the HFRs. I plan is to make these HFR values more accessible, e.g. perhaps plotting or tabulating them, in a future commit.