Fix signatures matrix declaration(s)

[baezortega]

While trying to run the code in the vignettes, the fit_signatures() function triggered an error related to the dimensions of the signatures matrix. While the definition of the matrix in sigfit_fit_stan.nmf is OK (matrix[S, C]), I think that there might be another definition, probably in the C++ code, that needs to be changed for signatures to be declared as a SxC matrix (instead of CxS).

set.seed(1)
mcmc_samples <- sigfit::fit_signatures(counts = mutations, signatures = sigs,
                                       iter = 1500, warmup = 500, seed = 1)

Error in new_CppObject_xp(fields$.module, fields$.pointer, ...) : 
  no valid constructor available for the argument list
trying deprecated constructor; please alert package maintainer
Error in new_CppObject_xp(fields$.module, fields$.pointer, ...) : 
  mismatch in dimension declared and found in context; processing stage=data initialization; 
  variable name=signatures; position=0; dims declared=(96,30); dims found=(30,96)
failed to create the sampler; sampling not done

[baezortega]

Fixed now