Parallelism: Fix or remove

[saulgreenberg]

Due to the bug reported in Issue #16, parallelism has been turned off by setting to only a single processor. The next step is to either:

• reintroduce parallelism after ensuring that duplicates are not created and images skipped on initial load, or
• remove parallelism ie, revert to a foreach

[saulgreenberg]

Reverted to ForEach instead of Parallel.ForEach when loading images. However, kept parallelism for Bulk DarkImages check (in DarkImagesThreshold.xaml.cs) as the consequences of errors there (if any) are minor.

After some investigation:

• Parallel.ForEach introduces occassional SQLite Database Locked messages
• could not track down what actually causes the issue (although its definitely in the Parallel.ForEach pass)