Avoid duplicate processing of flats

[simontorres]

The following problem presented;

• the only useful flats were obtained right after a science target, in which case the pipeline would take them as a flat specific for THAT science target leaving the rest of targets without a flat

As a workaround solution for this case was to process all flats at the beginning of the night, but for those targets (or even nights) when each science target has its own flats it will result in processing the flats twice.

Is not hard to implement.