Fix TypeError when using --cache_time argument

[tik0]

Running rosrun tf2_tools echo.py --cache_time 1.0 source_frame target_frame causes TypeError: time must have a to_sec method, e.g. rospy.Time or rospy.Duration, because the argument is not properly converted. The fix handles the conversion, if the argument was given.

[tfoote]

Thanks for catching this it's a good fix.
This is the minimum fix and looks fine.

Looking at the if statement I think it would be better to actually pass in the default cache time value to the arguments for better visibility.

It's in c++ here as tf2::BufferCore::DEFAULT_CACHE_TIME
We likely need to expose it into the python library. https://github.com/ros/geometry2/blob/noetic-devel/tf2_py/src/tf2_py.cpp and then reference that in the command line usage as a default here.