Feature: progress status for validation

[jscancella]

#Current behavior (if applicable)
when calling bag.isComplete(ignoreHiddenFiles) the system gives no feedback on where it is in the list of files to check

#Proposed behavior
When calling bag.isComplete(ignoreHiddenFiles) the system updates an object when it has completed checking a file with either success or failure.

#Why this feature is useful
This allows GUI tools to display a progress bar for the user. It would also be nice to be able to calculate and guess the time remaining but that might not be possible

#A small code example if possible

boolean ignoreHiddenFiles = true;
bag.isComplete(ignoreHiddenFiles, ProgressTrackerObject);

As far as what kind of interface the ProgressTrackerObject should have would depend on what GUI applications use (look into JavaFX and Swing)
See https://docs.oracle.com/javase/tutorial/uiswing/components/progress.html, https://docs.oracle.com/javase/8/docs/api/javax/swing/JProgressBar.html
and https://docs.oracle.com/javase/8/javafx/api/javafx/scene/control/ProgressBar.html