Backward compatibility

[tjibbed]

As part of a dashboard development I noticed the following:

RDS files created using older versions of the package don't allow for the node and ring colouring in the circular network plot, even when using the newer version of the package. This because the plotting function is stored in the HospiNet object, and the older plotting function doesn't recognise the extra arguments.

To solve this, I propose creating a version of the $new() function that just takes the entire object, copies the network information into the current HospiNet object, that does contain the current plotting function.

As an additions, I'd like to propose adding an attribute to the object that contains the version number, so any application using the package can check if the input object was created using the most recent version.

[tjibbed]

Note that my current workaround suffers from a bug reported in issue #28