Fix for issue #108: only if both parts of traj relay are running is 'motion possible'

This updates the conditions that need to be `true` before the *motion possible* field of the `robot_status` message is set to `true`: runtime status of the `ros` and `ros_relay` programs is now taken into account.

This should not introduce any incompatibilities.

Tested both in simulation and on a real R-30iA, HandlingTool, V7.70.

This might be a style thing, but I like the logic captured in nested IF statements, that way it's clear what logical statements evaluated.  In addition, the *safe* value (`false` in this case) should be set by default and only unset (`true` in this case) if the conditions are correct.

The reasoning is that the setting of the variable only happens in two places (this would make it easier to track down problems).  For example, I nearly missed that `this.mtn_psble_` is initially set in logic above.