Add verbose logging statements (optional)

[rmarkello]

Currently abagen.get_expression_data() takes a really long time (this should also probably be addressed...) and provides exactly zero information about its progress during that period. It would be fantastic to add some helpful logging / print statements to the function to let people running it know what's happening and how it's progressing.

At this point any status messages would be helpful, but am open to discussing what the optimal locations would be for providing status updates!