fix BITPIX value in generateSampleData to be 32 regardless of the CPU.

This is a fix for 64-bits machines where ints are interpreted as such by Python - Hence, generateSampleData creates arrays of 64-bit int - the BITPIX value in FITS file is 64 and not 32. This is a problem as some applications does not read this type of output - plus I don't think it's FITS standard. 

I have forced the values of dtype to be int32 and not CPU/platform-dependent.

Thanks,

James