This PR implements that change suggested in PR #884 and issue #901 regarding changing the system name when a transfer function is converted to a state space system and vice versa. Converted systems now have "$converted" appended to the system name. This is only done when you do an explicit conversation via ss or tf. Implicit conversions from a TransferFunction to a StateSpace object, for example as part of using interconnect use the original name.

In addition, prefix/suffix processing has been moved into the NamedIOSystem class, under _copy_names and _name_or_default methods.