Install ffmpeg
The "install_ffmpeg" function automatically downloads and installs ffmpeg to your computer. This function is ready to be tested on other operating systems. Added "land" and "water" arguments to make transition and make_transition2 functions to allow user to specify values for water and land in the transition layer. Useful if you want fish to transition overland to access receivers that are slightly on land from measurement error or low transition layer resolution. Also fixed #53 (closed) in interpolate_path.