#159 Add number of channels, integration time, total time in the FITS header

[Florent Mertens]

Currently, the header of the FITS files created by wcslean contains information about the image frequency bandwidth and the numbers of visibilities used in making the image (WSCNVIS), but it lakes the information about the integration time and frequency width of these visibilities. It would be nice to have these informations in the header as integration time and number of channels used. Having the total observation time would be a plus as well. It might be complcated to compute these values when making images from very hetergenous measurement set, so I would understand if you don't deem it possible.