Set new hypnogram format as default

[TomBugnon]

There is currently an inconsistency when saving hypnogram data:

the default button of the question box asking whether or not to use the new format is "No [use old format]", while the question text suggests the default should be "Yes":

`Since release 0.4, hypnogram are exported using stage "
"duration rather than point-per-second. This new format "
"avoids potential errors caused by downsampling and "
"confusion in the values assigned to each sleep stage. \n\n"
"Click 'Yes' to use the new format and 'No' to use the old "
"format. For more information, visit the doc at 

This PR simply changes the default button to "Yes [use new format]"

[raphaelvallat]

All good to me 👍