[Curve] Corrected the documentation for doxygen and added

 documentation for keywords

Documentation/ProjectFile/curve/i_curve.md

+Defines a single curve.
+
+The curve is given by a set of coordinates (the arguments) and a set of values
+at these coordinates. The values for any arguments between the given coordinates
+are obtained via linear interpolation.

Documentation/ProjectFile/curve/t_coords.md

+Coordinates at which the curve's values are given.

Documentation/ProjectFile/curve/t_values.md

+Values of the curve at the given coordinates.

MathLib/Curve/CreatePiecewiseLinearCurve.h

@@ -23,7 +23,7 @@ class ConfigTree;
 namespace MathLib
 {
 ///  Create a curve
-/// \param config   ConfigTree object has a tag of '<curve>'
+/// \param config   ConfigTree object has a tag of `<curve>`
 template <typename CurveType>
 std::unique_ptr<CurveType> createPiecewiseLinearCurve(
     BaseLib::ConfigTree const& config);