feat(tools): add function to convert beam index

ch_util/tools.py

@@ -2478,6 +2478,18 @@ def delay(
     return delays
 
 
+def beam_index2number(beam_index):
+    """Convert beam "index" (0-1023) to beam "number" (0-255, 1000-1255, etc.)
+
+    The latter, with 1000s indicating the beam's East-West index and the remainder
+    going from 0 through 255 indicating the beam's North-South index, is used in
+    the CHIME/FRB beam_model package."""
+    beam_ew_index = beam_index // 256
+    beam_ns_index = beam_index % 256
+    beam_number = 1000 * beam_ew_index + beam_ns_index
+    return beam_number
+
+
 def invert_no_zero(*args, **kwargs):
     from caput import tools
     import warnings