Enable FesomDataReader to have different source and target datasets (ecmwf#1046)

* Implement separate target and source files, adjust masking

* Fix casual masking

* Fix longitude conversion flag

* Fix casual masking strategy

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Co-authored-by: Seb Hickman <56727418+shmh40@users.noreply.github.com>

Author: kacpnowak

config/ifs_fesom_config.yml

@@ -9,7 +9,7 @@ ae_local_num_heads: 16
 start_date: 2000-10-10T00:00
 end_date: 2199-12-31T00:00
 start_date_val: 2200-01-01T00:00
-end_date_val: 2220-12-31T00:00
+end_date_val: 2209-12-31T00:00
 
 num_epochs: 111
 samples_per_epoch: 64

config/streams/fesom/fesom.yml

@@ -10,6 +10,7 @@
 FESOM_NODE :
   type : fesom
   filenames : ['ocean_node']
+  target_file: "/work/ab0995/a270088/Kacper/weathergenertor/AWICM3/ocean_elem"
   loss_weight : 1.
   source : null
   target : null

config/streams/fesom/fesom_elem.yml

@@ -10,6 +10,7 @@
 FESOM_ELEM :
   type : fesom
   filenames : ['ocean_elem']
+  target_file: "/work/ab0995/a270088/Kacper/weathergenertor/AWICM3/ocean_node"
   loss_weight : 1.
   source : null
   target : null