(court reverses lower court takings award worth millions of dollars; plaintiff, during life, had acquired Nevada ranch in 1978 and received permission from USFS and BLM to use 752,000 acres of adjoining federal land to graze cattle; takings case filed in 1991 and lower court determined that government took plaintiff's water rights by fencing off springs and streams and barring streambed maintenance; lack of evidence that government's actions barred plaintiff from using water for cattle and plaintiff lacked ownership interest in range improvements).