The ornamental design for a thermometer, as shown and described.
1. Thermometer
Fig. 1.1 is a bottom right perspective view of the thermometer; fig. 1.2 is a top right perspective view of the thermometer; fig. 1.3 is a bottom side view of the thermometer; fig. 1.4 is a left side view of the thermometer; fig. 1.5 is a top side view of the thermometer; fig. 1.6 is a rear side view of the thermometer; fig. 1.7 is a front side view of the thermometer; fig. 1.8 is a right side view of the thermometer; the blurred numbers disclosed in figures 1.2, 1.5 and 1.7 are only for illustrative purposes and form no part of the claimed design.