Changes to FlagHandler so that it can be used for Python only plugins

[natelust]

Add method to initialize FlagHandler with a vector of FlagDefinitions
Allow FlagDefinition to be constructed from a pair of strings
unit test testFlagHandler added

[natelust]

I am slightly worried what is going to happen when you have python strings mapped to c style pointers. If the python strings somehow get cleaned up. What will happen to the pointers. Is it practical to use std::strings, or am I being overly worried?

[pgee2000]

Good catch. I am not sure about the lifetime issue here, but it certainly should be thought about. Doesn't the same issue come put for the original addFields() call? Seems like what protects those pointers is that they are attached to a static member of the class which is using them. That means that someone abusing the original addFields() could probably create the same issue. But I propose that I adopt the same strategy with Python, and use a static list for the flagDefs and only call the addFields from the plugin constructor.

[TallJimbo]

This would indeed cause problems if called from Python. But the reason we needed to use raw pointers instead of std::string (direct initialization of an array of struts containing strings) has gone away in C++. Please try just switching to std::string.

[jdswinbank]

Please update the comment above to match the changes you've made below.