Fixed entropy calculations and more

[ashkjones]

fixes #51 and fixes #52

• Rows that are not assigned explicitly to an AOI are treated as belonging to a single "Undefined Area" AOI in the entropy calcs
• The "No AOI" label was changed to "Undefined Area" because now it is being treated as first-class AOI, i.e., it is considered an AOI in all calculations relating to AOIs
• I fixed a filtering bug I introduced in PR Fix saccade velocity sorting #53
• Moved header string constants and screen size to a package-private final class with static fields, called constants. This class cannot be instantiated and only acts as a place to share constants between other classes in the package rather than have each class re-declare its own copy over and over again. This will help if gazepoint ever changes their output format/headers; we only need to change the strings in one place.