Persist baseline image in calibration settings, bug fixes.

 - Fixed a bug where MSE was NaN for spectra without any absorbance at all
   (which resulted in a divide by zero error).
 - Added the baseline image path to the calibration settings data object so
   that the calibration image is persisted across creates and destroys
   (no need to capture baseline for every single sample).
 - Fixed bug where a double-tap on the Capture button caused a force close.