Improve fld_read time management for instantaneous fields

Context

fld_read routine always considers that the field timestep is centered on the middle of the time window separating two successive fld_read calls. This is usually true when the field is a time-averaged variable valid at the middle of the corresponding time bounds. But in the case of instantaneous fields, they are valid at the exact timestep of the fld_read call. Consequently, fld_read shifts the field valid timestep by half its frequency, which is wrong and can significantly offset atmospheric forcings for example.

Proposal

introduce a new prefix (none, + or -) to the variable name in fld_read structure to specify whether the field read by fld_read is (time-)averaged or instantaneous, and shift field valid timestep accordingly in fld_read (up or down according to the sign).