Add detector response fitting tool; many small fixes.

Add tool to make detector response function (DRF) from characterization data; tutorial on use, and some example spectrum files here.
The included generic NaI detector response functions had some efficiencies that were not correct for some energies; this has been fixed.
Fixed crash on macOS when file or directory was selected to be opened.
Fixed issue downloading CSVs files on macOS.
Added a number of new spectrum file formats, and accommodated some variants of other formats.
Added option to automatically load a DRF from a given detector, or model of detector.
Many smaller bug fixes and improvements.