Main and 9 other classes: fixed several FindBugs issues

**mvn findbugs:check**

Main:
- Created the dateFormat field to use instead of the static Constants.DATE_FORMAT field.
- Return a copy from the date objects from the getFromDate and getTillDate methods.

ChooseMetricsForm:
- Iterate over the entrySet instead of the keySet (and using metricsMap.get) in the createListModels method.

DataEntryForm:
- Created the dateFormat field to use instead of the static Constants.DATE_FORMAT field.

DetailsFrame:
- Made the DetailsTableCellRender inner class private and static.

ProgressLogReader:
- Changed the static LOG_FILE_DATE_TIME_FORMAT field into the non-static logFileDateTimeFormat field.

ReportUnitComparator:
- Changed the static DTF field into the non-static dateTimeFormat field.

Constants:
- Removed the DATE_FORMAT and MEASURED_DATE_FORMAT fields.

PropertyFileWriter:
- Changed a String variable into a StringBuilder in the updateMetricsSelection method.

Utilities:
- Created the createDateFormat method.

ReportUnitTest:
- Created the measuredDateFormat field to use instead of the static Constants.MEASURED_DATE_FORMAT field.