Add TF Prefix to effort plugin

[ahoarau]

Simple PR to add a TF Prefix property to the Effort Plugin.
Use case : /robot1/robot_description + tf_prefix set to robot1

Also fixes #892.

[dhood]

Thanks for the patch, @ahoarau

@tfoote I have read that the tf prefix was causing headaches in tf1, and I've seen errors before about e.g. base_link not resolving to /base_link well. Would you be able to comment on the impact of prepending a forward slash to a frame where it didn't otherwise have one? I'm thinking we should only add the slash if the tf_prefix is specified, to minimise impact.

[ahoarau]

You are absolutely right, we should not add leading slash if no tf_prefix is present.
Let's change it for :

std::string tf_prefix = tf_prefix_property_->getStdString();
std::string parent_link_name =  (tf_prefix.empty() ? "" : tf_prefix + "/" ) + joint->child_link_name;

Very glad to hear that tf_prefix is meant to disappear, but I'm not sure how I will spawn multiple robots in the same master. Multimaster is quite a big overhead :)

[dhood]

That change looks reasonable, but for clarity the variable should now be named tf_frame_id as opposed to parent_link_name

btw this change will be held for ROS Melodic since it will cause an ABI change

[dhood]

I broke out the non-abi breaking change into #1229 so it can be released into indigo/kinetic

[dhood]

@ros-pull-request-builder retest this please.

[wjwwood]

@dhood merge or comment

[dhood]

thanks for iterating @ahoarau