Getting constellation size as a variable

[Austen, Jeffrey]

When using the Constellation Object block in Gnu Radio Companion how can 
one obtain the size of the constellation, M, or the number of bits per 
symbol? This value is needed so other blocks, e.g. Pack k Bits, can be 
adjusted accordingly when the constellation size is changed.

A simple example would be "Random Source" producing a binary output, 
followed by "Pack k Bits" to create symbols, then "Constellation 
Encoder" to convert the symbols to complex-envelope symbol values. 
Currently when the constellation size is changed it is necessary to also 
change an additional variable, the number of bits per symbol (which is 
needed by the Pack k Bits block). I'd like to be able to get this value 
from the constellation object.