SET DEFAULT PARAMETERS !
If not specified in the input file these are the default options that will be set for all parameters under the namelist ¶meters_physics'.
READ INPUT OPTIONS !
Overwrite any default options with those specified in the input file. Then change the other parameters consistently.
CHECK BACKWARDS COMPATIBILITY !
Make sure stella either runs or aborts old names for variables or namelists are used
BROADCAST OPTIONS !
Broadcast these parameters to all the processors - necessary because the above was only done for the first processor (proc0).