wavtospice.py: Add cast to float to avoid downcast to integer.

Running this on my system resulted in an output file with integer values in the second column. Adding a cast to float seems to have fixed the issue for me. My version of Python is:

Python 2.7.6 (default, Feb 26 2014, 12:07:17) 
[GCC 4.8.2 20140206 (prerelease)] on linux2

Thanks for the useful tool, by the way. I'm just getting in to circuit simulation with ngspice and this was a time saver. Cheers!