Reduce memory usage when calculating var-bound for Bouchard variants

Do the scaled sum of the log_one_plus_exp of a matrix in a single
(Cython) for loop, instead of creating a copy of the matrix with the
log_etc result then summing over it.
Stupid silly bug in the launch script saw us set the runtime to zero
Stupid bug with for-loop overwriting the list of elements to loop over
Silly bug in main script, typo in variable name
Failed to alter all instances of a renamed variable (NUM_TOPICS --> TOPIC_COUNT)
Ensure that no element of vocab is ever zero

It screws up queries if it is
Wrong parameterisation of the log_likelihood function