Change default tricera path to assume that tri is in $PATH

[woosh]

This PR modifies the default for option -saida-tricera-path to tri. Effectively this assumes that tri is in scope of $PATH. This is a more convenient default than ~/Documents/tricera/tri since it can easily be achieved by either adding to $PATH or setting up symlinks.

[gustavung]

I agree with this change. We should not expect any specific layout in the file system. If a user does not wish to have Tricera in the path, it is always possible to provide a custom location.