Added a vector type to the function attributes. Re #7623.

Modified the GUI to handle the new type. Created BSpline fitting function which uses the new vector attribute to test it in the GUI.
Not all compilers recognize front() and back() of std::string.

Re #7623
Fix compiler warnings. Re #7623
Reverted setting of function attributes before parameters

in FunctionFactory and FitPropertyBrowser because it breaks existing code. Re #7623
Merge branch 'feature/7623_vector_attributes' into develop