Make posible to work with PyThea in "offline" mode

[AthKouloumvakos]

Every time that PyThea loads an event or a fitting file, it searches VSO for data. If there are no newer data, it skips the downloading part and continues with the image loading. This makes the process completely dependent on a network connection.

It could be a useful feature to make an "offline" mode. In this case, if the data for an event have been downloaded, the app will automatically skip the Fido search and it will load directly the available images.

This step to complete, might need some sophisticated handling on the downloaded files, e.g. to database the fits files.