MATLAB representation of JSON numeric array; speedup

Hi Christian,

I noticed that simple json arrays (of type double/int/bool) are converted to MATLAB cell-arrays. This is inconvenient in most cases. Also, when reading a large json-array (I had 5 million entries), the process of creating the cell elements in the C-code takes several seconds.

The proposed change creates MATLAB matrices for all-numeric arrays. Copying data from the json-c structures to MATLAB becomes much faster.

Cheers!