Dynamic orbital parameters do not use current year and default should be NFIXORB=1

[jhardenberg]

The default is currently NFIXORB=0 so that dynamic calculation of orbital parametres is active. This uses N_START_YEAR fron plasim_namelist to determine the year for which to compute orbital parameters. The problems are:

1. This should be dynamic and change from year to year. Currently, using N_START_YEAR orbital parameters are indeed fixed also in this cas to those of the initial run year.
2. By default our N_START_YEAR is 1 ! So actually orbital parameters for year 1 AD are used.

[jhardenberg]

Proposed solutions:

1. Set NFIXORB=1 by default in planet_namelist (this is probably expected behaviour anyway)
2. Set default N_START_YEAR (in namelist plasim_namelist) to 2000 (in order to have decent present-day orbital parameters by default and to make clear to the user that this parameter is not entirely irrelevant)
3. Make sure that radmod.f90 uses the current year and not just the start year when updating orbital parameters

[jhardenberg]

All changes have been implemented in branch fixorb.
Everything now seems to be working as expected. Let's do some more tests though.

[jhardenberg]

The current implementation works by updating orbital parameters (if NFIXORB=0) every time plasim/radmod is initialised. In the case of runs with chunks of 1 year in length (the default created by most.x if the GUI is not selected), this happens every year, since N_RUN_YEARS=1,the code runs for 1 year, all outputs are saved / stored/converted and then the next year is performed starting from plasim_restart. When the GUI is selected in most.x, it sets instead N_RUN_YEARS to the total desired length of years to run and most_plasim_run is set for only one chunk. This also means that the orbital parameters are updated only at the beginning od the first year.
I leave this just as a warning, I would consider it expected behaviour (but needs to be better documented)