new S/R GET_PERIODIC_INTERVAL to calculate which time-record (and linear  interpolation weights) to use in a periodic time-series ; do not divide by  time-step to convert to integer:  - this fixes case where externForcingPeriod is not multiple of time-step     (Thanks to Christopher L. Wolfe)  - this fixes case where externForcingPeriod is an odd multiple of time-step    (was wrong by half of time-step); update results of tutorial_global_oce_in_p. store index of time-record which is currently loaded:  - works without any assumption on sequence of calls to EXTERNAL_FIELDS_LOAD ;  - needed for the adjoint but not yet used because of extensive recomputation;    (to be fixed soon)
new S/R GET_PERIODIC_INTERVAL to calculate which time-record (and linear
 interpolation weights) to use in a periodic time-series ; do not divide by
 time-step to convert to integer:
 - this fixes case where externForcingPeriod is not multiple of time-step
    (Thanks to Christopher L. Wolfe)
 - this fixes case where externForcingPeriod is an odd multiple of time-step
   (was wrong by half of time-step); update results of tutorial_global_oce_in_p.
store index of time-record which is currently loaded:
 - works without any assumption on sequence of calls to EXTERNAL_FIELDS_LOAD ;
 - needed for the adjoint but not yet used because of extensive recomputation;
   (to be fixed soon)

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