Use 'logging' module instead of 'print'

[aperrin66]

In several places, ´print´ is used for log messages (for example in the constructor of the ´Nansat´ class). Loggers should be used everywhere, so that the destination (file, console, etc...) can be easily configured. This will become more of a problem as we are going to use ´nansat´ in long-running applications, and not only in a "desktop environment".