Added basic functionality to run multiple cases.

CanSen can now run multiple cases given by the input file. When the
appropriate CLI option ('--multi') is given, CanSen will parse multiple
cases from the input file and run each independently. Rather than print
detailed output and save data for each, the output file now contains
only the calculated ignition delays and initial conditions for each (on
separate lines).

The next step is to wrap this in a multithreading environment so that
each case can be performed in parallel.
Added --multi option to usage documentation
Moved redirection of stdout outside of case loop
Added multiprocessing support to perform multiple cases in parallel