COFFE should check for unsupported LUT sizes in the input files.

[vaughnbetz]

Asking for a fracturable 4-LUT currently crashes COFFE. We should give a good error message in this case.
COFFE knows how to build 4 LUTs, 5 LUTs and 6 LUTs.  But if you specify that you want fracturable LUTs, it builds a LUT one size smaller and then wires those together.
So a fracturable 4-LUT needs to build two 3-LUTs, but the code won't build a 3-LUT and crashes. 

See 

COFFE/coffe/fpga.py

Line 1061 in 7dca438

init_tran_sizes = self._generate_4lut(subcircuit_filename, min_tran_width, self.use_tgate, self.use_finfet, self.use_fluts)

It would be good to put error messages in to check for this. 

• We should give an error (add an else clause) in the line above so you get a better crash message (right now we just roll on through to an uninitialized variable). 
• And then the real check would be to check for inputs that violate this constraint and give a good error message at input parse time.  
• Should also test LUT sizes > 6 and < 4 and make sure you get a good errors message (K = 8 does give an unsupported parameter error; maybe could be more user-friendly though).