Add additional detail about spawn and settlement month potential conflicts

[e-perl-NOAA]

Issue Description: User Communication A recent communication with a SS3 user indicated that a fatal error occurs when specifying a non-January settlement month. The warning message is: "Cannot have spawn_time_seas after beginning of a season and settlements in the same season put spawning at beginning of the season, or move settlements to next season". The user said that their reading of the manual made them believe that you can specify the spawn month and settlement month.

The user also noted that the model will run if they set settlement time to January and age at settlement at 0 or 1. If they set settlement age to 0 when spawn month is May, they get a warning that "settle_month is less than spawn_month, so logical age at settlement calculated to be: 1 for settle_time 1. Does not match read value of 0 are you sure?". The user asked iff settlement age is set to 1, are recruits now age-1s rather than age-0?

Response to User @iantaylor-NOAA responded to the user with: The basic concept here is that you don't want new recruits settling prior to when they were born. The tricky part is accounting for the timelines of spawning/hatching/settling/recruiting/etc. and then mapping onto that an integer age and a real age for each cohort for purposes of modeling growth, calculating expected age comps, etc. For the warning message is: "Cannot have spawn_time_seas after beginning of a season and settlements in the same season put spawning at beginning of the season, or move settlements to next season", @iantaylor-NOAA was able to track down a previous Vlab issue saying the following:

• If spawning is at the beginning of a season, then settlements can happen at the beginning of that season or anytime thereafter.
• However, if spawning happens within a season, then that spawning is not internally calculated until the end of that season (to take into account within season mortality) so recruits from that spawning cannot happen until the beginning of the next season.
• spawn_month=6 and settlement_month=7 violates the second relationship above in an annual model. It would be OK in a model with two six month seasons
• spawn_month=1 and settlement_month=7 and age_at_settlement=0 is OK
• spawn_month=6 and settlement_month=1 (e.g. the following January) and age_at_settlement=1 is also OK

@Rick-Methot-NOAA further responded that the key consideration on the last item is for them to discuss with the age readers to know whether they would call that fish a "1" or a "0". SS3 provides flexibility to assign age at settlement. So, spawn_month=6 and settlement_month=1 (e.g. the following January) and age_at_settlement=1 is also OK. From a spawn_month=6, you might have one settlement event in Dec at age 5 months with age at settlement = 0, and a settlement event in Jan at age 1. SS3 will track them both as the same year class because the Dec setters become age 1 on Jan 1.

@Rick-Methot-NOAA also provided the Settlement Timing and Growth.docx in the comments below.

Action needed from SS3 team There needs to be additional documentation in section 8.4.2: Settlement timing for recruits and distribution of the manual to help users navigate these types of situations. The user suggested a timeline of events for a default model structure as it might help people visualize the timing of the calculations.

[e-perl-NOAA]

@iantaylor-NOAA and @Rick-Methot-NOAA, if you have any suggested text to add to this section, let me know and I can add it.

[Rick-Methot-NOAA]

Here is an old, but relevant, doc that never quite made it to completion. Settlement Timing and Growth.docx

[e-perl-NOAA]

Settlement timing doc that Rick provided added to ss3 website.