The ornamental design for a fountain as shown and described.
FIG. 1 is a perspective view of a fountain showing my new design;
FIG. 2 is a left side view of the fountain of FIG. 1;
FIG. 3 is a right side view of the fountain of FIG. 1;
FIG. 4 is a front view of the fountain of FIG. 1;
FIG. 5 is a rear view of the fountain of FIG. 1;
FIG. 6 is a top view of the fountain of FIG. 1; and,
FIG. 7 is a bottom view of the fountain of FIG. 1.
The broken line showing of a portion of the fountain in FIGS. 1 and 6 are included for the purpose of illustrating and forms no part of the claimed design.