Removed old test files
Sim::get: Enabled aborting on bad variables.

This should avoid mistakes when using SimKernel, as the value of variables which should have been read but could not be found (when no default was specified) is undefined.
Added more detail to tutorial Program.
Output of boolean values is now True and False.

The most common use case are outputs like this: "Sim: It 1: Warning: Cannot get value for Some_Boolean_Variable. Using False instead!" SimKernel would have used "0" and "1" before.
When using default value, output is less dramatic.

It now looks like this: "Cannot get value for Something. Using 1 instead."
Corrected typo in error message.
Minor bugfix: Function could end w/o return call.

This was causing compiler errors like "SimKernel/expression_sim.h:159:7: warning: control may reach end of non-void function".
Merge branch 'master' of git://github.com/olavolav/SimKernel