Update C algorithm test code.
Update main.c with corrected code from data_types.c

I used data_types.c to test the oversampling and conversions.  I
thought that I had the correct process, especially on the part
about shifting the values to put the midpoint at zero when cast to
signed integers.

This may not be correct on all platforms due to type casting rules,
but data_types.c works as I expect on an x86 system, compiled with
gcc.