Return the number of sub-arrays in the current observation.
EXAMPLE
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0>print "There are", nsub, "sub-arrays in the current observation."
There are 4 sub-arrays in the current observation. 
0>

CONTEXT
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nsub() is a function that returns the integer count of the number of
telescope sub-arrays per IF of the current observation.

RELATED FUNCTIONS AND COMMANDS
------------------------------
nif    -  Return the number of IFs in the current observation.
nchan  -  Return the number of spectral-line channels per IF.
nbase  -  Return the number of baselines in a given sub-array.
ntel   -  Return the number of telescopes in a given sub-array.
header -  Displays the characteristics of the current observation.