Mosel instances management

The XPRDconnect function starts a Mosel instance and returns a XPRDmosel object. The method used to create this instance depends on a connection string that is interpreted in a similar way as with the connect function of the mmjobs Mosel module: three I/O drivers can be used to launch a Mosel instance. The first one, "rcmd:", executes a command in a separate process—typically this will be directly mosel or a special command to start Mosel on a remote host (e.g. the command ssh). The second driver, "xsrv:", requires the xprmsrv Mosel remote launcher to run on the target machine: the connection is established with such a server through a TCP link. The last driver, "xssh:", is similar to the previous one but establishes the connection to the server through a secure SSH tunnel. The handling of the tunnel is achieved by a separate process: by default, the xprmsrv program is used but optionally another SSH client may be selected using XPRDsetsshcmd.