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.

See merge request !140