[WIP] ComponentPath and ChannelPath properties for Sockets and Inputs.

[chrisdembia]

@aseth1 , @klshrinidhi and I just chatted about changes I had initiated to convert the property within Sockets and Inputs to use ComponentPath and ChannelPath instead of std::string. The primary goal of these changes for 4.0 is to help fix bug #1420, but the changes also provide substantial cleanup of the code. The plan is for @klshrinidhi to finish this PR by tackling the tasks listed below, to get this PR merged, and then to fix #1420 in a separate pull request. If finishing these changes takes too long, then we should just fix #1420 the quick way and finish these changes after 4.0.

To-do list:

• Write documentation for the ChannelPath class.
• Some simple python/MATLAB tests to make sure it's easy to use ComponentPath/ChannelPath with Component methods (e.g., Component::getRelativePath()).
• Get rid of AbstractSocket::checkConnecteeNameProperty(); error checks should now occur in the deserialization code in Property.h (readSimplePropertyFromStream()).
• Create a base class for Sockets and Inputs whose template argument is the property type (either ComponentPath or ChannelPath); this base class would have string-based methods like setConnecteePath(std::string) and appendConnecteePath(std::string).
• Create new methods on AbstractSocket and AbstractInput that use the path type (e.g., setConnecteePath(ComponentPath)), and use these in Input::findAndConnect(), etc.
• Get rid of the AbstractSocket constructor that takes isList as an argument; this should be obtained by the new base Connector by asking the property if it is a list (I can clarify this in person).
• Decide if Channel::getPathName() is unnecessary (replaced by getAbsolutePath() and getRelativePath()) and if so, delete it.
• Properly handle errors within operator>>(std::istream& in, ComponentPath&)
• Remove the parseConnecteeName() helper function, and related tests.

@klshrinidhi please feel free to chat with me about these changes.

See the branch ComponentPath_property_sandbox for my first shot at rearranging the class hierarchy for Sockets and Inputs.

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