Add a method to check if a path exists

[cchampet]

The path could be a file/folder/link/sequence.

[cchampet]

Something like:

bool exists(const std::string& pathStr)
{
    if(bfs::exists(pathStr))
		return true; // file, link, or directory
    Sequence outSequence;
    return browseSequence(outSequence, pathStr);
}

But browseSequence seems to not work...