347 dependence on graphviz

This addresses #347.

Dependence on graphviz had crept back into the main program some time ago via "runtime graphing" (generating a graph of resolved dependencies on the fly). This commit fixes that problem so that we can run suites even in graphviz is not installed, and does the same for gcylc (i.e. gcylc will run with the graph view disabled if graphviz is not installed).