Can now calculate descriptive statistics with values obtained from
manual data entry.
Implementation added for manual data entry. Some minor refactoring.
Refactoring from SimpleMenuBar to MenuBar.
SimpleMenuBar refactored to MenuBar.
Removed unused action listeners. Wrapped some long lines. Added
exception handling for NumberFormatException, and added safe guard when
significance level is not within a valid range.
Minor improvement to output.
Line wrapping and removal of unused action listeners.
Removed some comments. Wrapped some lines. Added documentation. Some
refactoring of variables.  Added rejectNull boolean variable for
conclusion of hypothesis test.
Refactoring. Changed testHypothesis to private, so that it is invoked
within constructor of OneSampleTTest. toBigDecimalArray is no longer a
method, but rather a part of stringToBigDecimalArray.
ZScore improved. Documentation added. Z score and P value now formatted
to four decimal places.
Removed comments.
Refactored some variables.