String conversion helpers

We define a number of string conversion helpers here just so that we don't require each type that we may want to instantiate into a ModelParam derivative to define operator<< and operator>>. The main reason is that this definition requires including <sstream> (especially if the class is all-inline like Point2D<int>), which would largely increase the compile time if we did it for all our simple types like Dims, Point2D<int>, etc. But the code below may some day migrate into proper definitions of operator<< and operator>> for those data types.

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