Adds alerts to inform the user of Exceptions/Errors that occurred while
parsing a ZIP file, instead of silently failing and closing the
openFileDialog window.

Use a BufferedInputStream for counting the bytes of a ZipEntry to reduce
the number of read operations on the hard drive.

Adds a HashMap to allow CityDoctorController to remember the Checker of
a CityDoctorModel, to allow switching of models with ZipEntryManager
without having to rerun the Checks when an already validated model is
loaded again.

Short-term meaning, that the memory is only kept for this session and
only till the next CityGML source is loaded (even if the same zip file
gets loaded again).