First version of main() using a parameter file to get constants.

That's pretty big but does not touch the physics, the parallelisation nor MPI.

In brief:

• Latest version of the parser from @jwillis
• More uniform name of functions in the unit module.
• Added the ability for the engine, space, unit and partition objects to construct themselves from a parsed parameter file.
• Added an example yaml file with full documentation.
• Added yaml files and run scripts for the Sedov, Sod and uniform examples.
• General tidying of the main().
• Dropped the dependency on z-lib which we don't use.
• Updated the README to reflect the new command-line parameters.
• Cleaned-up the main() such that the only command-line parameters read in are policies and debugging options.
• Added the option to do a 'dry run'. This reads the parameter file, allocates the memory required by the ICs but does not read anything and constructs a space and engine. This allows us to check the validity of a given parameter file.

Happy to guide you through the changes if need be.

This closes #81 (closed).