Add time_to_think to increase survivability

Add time_to_think to increase survivability

I thought that the program should be a tool to demonstrate the dangers
in parallel computing. Hence I let the user wholly define what
philosophers can do.
This point of view has however failed me multiple times during
evaluation... This (and a couple of conversations) led me to believe the
only way to pass is to add another element - the time_to_think.

When odd number of philosophers is entered and the time_to_sleep is too
low, the time_to_think is set to such a value that philosophers can
share the forks fairly.