Fix unsequenced warning in pyne.cpp

This warning currently only shows up with clang right now (I think).
This may be slightly slower depending on how intelligently the compiler
unrolls the loop.
throw error if mcnp_to_id is unsuccessful
hsize_t is unsigned long long so < 0 check doesn't work
Remove these statements since data_dims is unsigned
replace numbers > 2^16 with INT_MAX
Add null return
Expand loops to determine power of 10 in pyne.cpp

This unrolls the loops (and gets rid of using pow) but keeps the
bitshift assigns seperate so that the order is defined
initialiaze cv to 0,0 to get rid of warning