Fix new DAMP convention

[giacomomagni]

In #140, we have changed the convention for DAMP: 0, but this line has not been fixed accordingly.

pineko/src/pineko/fonll.py

Line 238 in 68c41e7

is_damped = damp != 0

It should be replaced with:

 is_damped = damp != -1

[alecandido]

Just to mention it: in a sense, a common part would be to put

 is_damped = damp > 0

but I believe that being more specific would be better (in case you made an error somewhere, there are more chances that you can predict the outcome).