TensorBoardOutput's resource is not cleaned up nicely

[zhanpenghe]

When using the logger with tensorboard, it will hang if we don't call logger.remove_all(). Here is a simple snippet for reproduce it:

import dowel
from dowel import logger

logger.add_output(dowel.TensorBoardOutput(log_dir))

This is mostly due to tensorboardX.SummaryWriter failed to clean up its resource.