Adds letkf.yaml(.j2) (#1134)

This PR adds the initial letkf.yaml (which is good enough to get
partially ingested by the letkf executable)

One step towards resolving
#1091

parm/soca/letkf/letkf.yaml.j2

@@ -0,0 +1,69 @@
+geometry:
+  geom_grid_file: soca_gridspec.nc
+  mom6_input_nml: mom_input.nml
+  fields metadata: fields_metadata.yaml
+
+time window:
+  begin: &date '{{ ATM_WINDOW_BEGIN }}'
+  length: PT6H 
+
+background:
+  members from template:
+    template:
+      date: '{{ ATM_WINDOW_MIDDLE }}'
+      ocn_filename: enkfgdas.ocean.t06z.inst.f009.nc
+      ice_filename: enkfgdas.ice.t06z.inst.f009.nc
+      read_from_file: 1
+      basename: ./ens/mem%mem%
+      state variables: [socn, tocn, ssh, uocn, vocn, cicen]
+    pattern: '%mem%'
+    nmembers: {{ NMEM_ENS  }}
+
+observations:
+  observers:
+
+driver:
+  do posterior observer: true
+  save posterior mean increment: true
+  save posterior mean: true
+  save posterior variance: true
+  save prior mean: true
+  save prior variance: true
+
+local ensemble DA:
+  solver: LETKF
+  inflation:
+    rtps: 0.5
+    rtpp: 0.6
+    mult: 1.1
+
+output:
+  datadir: data_output/
+  date: *date
+  exp: letkf
+  type: ens
+
+output mean prior:
+  datadir: data_output/
+  date: *date
+  exp: letkf
+  type: fc
+
+output variance prior:
+  datadir: data_output/
+  date: *date
+  exp: letkf
+  type: fc
+
+output variance posterior:
+  datadir: data_output/
+  date: *date
+  exp: letkf
+  type: an
+
+output increment:
+  datadir: data_output/
+  date: *date
+  exp: letkf.inc
+  type: an
+