Add a smal change to bugfix map chans for seviri to work correctly.

[orlewis]

When final testing was carried out on Seviri the VAR statistics did not look correct. This problem was only happening for channel 6.

The bug comes about from seviri having 6 channels read in but only 5 output to the varobs. This meant that a loop which cycled on the number of channels went round 6 times. A variable called arrayindex was used to assign where in an array to palce a value, this is initially set as the loop number. For seviri the logic follows a route where the arrayindex was updated to be the actual channel number, however on the final loop 6 the if logic prevents the arrayindex being reallocated and so channel 6 gets filled with the final channel 11 observations.

This is fixed by setting an exit on the loop if the condition is not met.

[orlewis]

KGO have been run successfully http://fcm1/cylc-review/taskjobs/olewis/?suite=sith_kgo_231219

Geos can also be seen here:

SEVIRI: https://www-nwp/~plevens/ngops_boitenoire/sith_SEVIRI_test/seviriclr/glu_20210701T1200Z/ ABI: https://www-nwp/~plevens/ngops_boitenoire/sith_ABI_test/abiclr/glu_20210701T1200Z/ AHI: https://www-nwp/~plevens/ngops_boitenoire/sith_AHI_test/ahiclr/glu_20210701T1200Z/