implement simple (obliquity only, assuming circular planet orbit)  seasonal cycle for incoming insolation ; used when run-time parameter  "select_incSW" is set to 1 ; by default (=0), use old unchanged insolation.
implement simple (obliquity only, assuming circular planet orbit)
 seasonal cycle for incoming insolation ; used when run-time parameter
 "select_incSW" is set to 1 ; by default (=0), use old unchanged insolation.

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