Expands StackSearch to support single searches

[jeremykubica]

This is step 3 of N in addressing #447.

Expands the StackSearch class to support single searches. In addition, this cleans up data management within StackSearch:

• The combined PsiPhiArray is maintained and used for all processing in place of the individual vectors of RawImage, providing a single consistent data source for all functions. The major change here is extracting the psi and phi curves from the PsiPhiArray instead of the individual images.
• Setters for min_obs and min_lh are added to StackSearch
• Additional null pointer checking is added to functions for safety.
• The helper function make_trajectory is used instead of manual Trajectory creation in tests.
• Add a bunch of validity checks for the parameter settings functions.
• Add some doc strings.

Passes all tests and the diff test on some old DEEP data.