Calculating the Thermodynamics of Weakly Hydrogen-Bonded Complexes from Heteronuclear NMR Data: Base-Pairing Stabilities of a 5-Methyl(^1^5N~2)[O^2,O^4-^1^7O~2]uridine ( = (^1^5N~2)[O^2,O^4-^1^7O~2]-Ribosylthymine) Derivative, and Structural Implications