Handle zeros gracefully

[philmikejones]

Handle zeros in constraint tables gracefully

[philmikejones]

0s in cons done; what about 0s in aggregaged inds?

[philmikejones]

Error reported by Derrick Atherton by email October 2017:

Error in ipfp::ipfp(x, t(ind_cat), x0 = rep(1, nrow(ind_cat)), maxit = iterations) : 
  BLAS/LAPACK routine 'DGEMV ' gave error code -6

Able to replicate the error when an entire zone has an empty population (i.e. all zeros)