NR UE: introduce support to define multiple cells

This feature shall allow in future to simulate an arbitratry number of UEs connected to an other arbitrary number of cells.
Therefore each RF-frontend (also called RU) needs to be configured for one specific cell.
In case of handover of a UE from one cell to another cell, it needs to switch RUs accordingly (TODO).

There has to be a 1-to-1 mapping between RUs and cells.
Each UE will be assigned to one of the defined cells.
Initially all UEs are assigned to cell 0.

ci-scripts/conf_files/nrue.uicc.ntn-leo.conf

@@ -29,6 +29,18 @@ RUs = (
     }
 );
 
+cells = (
+    {
+        ru_id      = 0;
+        band       = 254;
+        rf_freq    = 2488400000L;
+        rf_offset  = -873500000L;
+        numerology = 0;
+        N_RB_DL    = 25;
+        ssb_start  = 60;
+    }
+);
+
 #/* configuration for channel modelisation */
 #/* To be included in main config file when */
 #/* channel modelisation is used (rfsimulator with chanmod options enabled) */