CCPPize prescribed_strataero (stratospheric aerosol forcings) (CAM6, CAM7)

Need to make `prescribed_strataero` in current CAM into a CCPP scheme for CAM-SIMA. Will not be ported back to CAM, like the other `tracer_data`-using prescribed schemes.

This is a CAM6, CAM7 work and for MAM (see build-namelist), so deferred to a future date.

```perl
# turn on stratospheric aerosol forcings in CAM6 configurations
my $chem_has_ocs = chem_has_species($cfg, 'OCS');
if (($phys =~ /cam6/ or $phys =~ /cam7/) and $chem =~ /_mam/) {
# turn on volc forcings in cam6 -- prognostic or prescribed
if ( $chem_has_ocs ) {
# turn on prognostic stratospheric aerosols
add_default($nl, 'modal_strat_sulfate','val'=>'.true.');
} else {
# turn on prescribed stratospheric aerosols
add_default($nl, 'prescribed_strataero_feedback','val'=>'.true.');
add_default($nl, 'prescribed_strataero_3modes','val'=>'.true.');
}
}
if ( $nl->get_value('modal_strat_sulfate')=~ /$TRUE/io and
$nl->get_value('prescribed_strataero_feedback') =~ /$TRUE/io ) {
die "Cannot set both modal_strat_sulfate and prescribed_strataero_feedback to TRUE \n";
}
```