interp2.m: Add input validation for strictly monotonic x,y (bug #41838). * interp2.m: Add input validation for strictly monotonic x,y (bug #41838). Move input validation of x,y out of individual method blocks and into common validation section at start of function. Don't unnecessarily create meshgridded XI, YI for "spline" method. Add %!tests for strictly monotonic inputs.

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