Add absolute tolerance optional input (default: 0).
Modify the spike monitor class to use as stopping criteria:
         ||r|| < absTol + relTol * ||r_0||
Add driver options for specifying absolute tolerance (default = 0).

Use options --relTol=TOL and --absTol=TOL to specify the relative and absolute tolerances, respectively.
For backward compatibility, relative tolerance can still be specified using '-t' and '--tolerance'.
Merge branch 'feature/tolerance' into develop