Improve algorithm for switching to crowded field kernel for HAP segment catalogs

[stscijgbot-hstdp]

Issue HLA-1427 was created on JIRA by Rick White:

Recently I have discovered some images where the HAP software used the Gaussian kernel, but the RW2D kernel would have been a better choice. This innerspace page shows examples of the issues with the current scheme and discusses the thresholds that are used to switch to the crowded-field kernel.

These issues can be fixed with a new threshold test that should improve the decision of whether to switch to theRW2D kernel. The test is very similar to the current test that sets a threshold on the percentage of the image covered by the biggest segment region. But instead of the percentage size of the biggest island, it uses the size of that island in pixels. That is already calculated (and printed) in the code. A single threshold value of 25000 pixels will work for all instruments and detectors. In order to minimize the changes, I propose adding this new test alongside the existing algorithms rather than modifying the existing algorithms or parameters.

This is a relatively minor code change but should improve the HAP catalog quality in approximately 15% of the fields.

I think this can be integrated with the other recent HAP catalog changes fairly easily. In particular, the testing for the new modifications can be combined with the testing for this addition.