Add in new option for CMIP7 1850 aerosols

We have a preliminary dataset for CMIP7 1850 aerosols. So we should add an option to DATM_PRESAERO for CMIP7 1850. Something like

clim_cmip7_1850

See how this was done in CAM for this B simulation:

``` fortran
&prescribed_strataero_nl
prescribed_strataero_cycle_yr = 1850
prescribed_strataero_datapath = '/glade/derecho/scratch/tilmes/archive/b.e30_alpha07c_cesm.B1850C_MTt4s.ne30_t232_wgx3.251.001/atm/proc/ozone_strataero_day_5'
prescribed_strataero_file = 'ozone_strataero_b.e30_alpha07c_cesm.B1850C_MTt4s.ne30_t232_wgx3.251.001_1850climo_zm_5day_c251204.nc'
prescribed_strataero_type = 'CYCLICAL'
prescribed_strataero_use_chemtrop = .true.
```

https://github.com/NCAR/cesm_dev/issues/264