Add more link APIs, with tutorial

[chapulina]

• Add a migration tutorial for links
• Add a few new functions to the link class
• Remove Model::Parent because I just realized we have ecm.ParentEntity. We need to be careful to merge this before the other one is released.
• Added TODOs to the empty items on the table, because leaving it empty doesn't render well, see https://ignitionrobotics.org/api/gazebo/3.3/migrationmodelapi.html

Part of solving #325

[claireyywang]

LGTM with minor comments. Probably need to rerun CI though

[claireyywang]

Is comp->Data() always going to be true if set, or can it be set as true or false? The logic here might be confusing since we don't immediately know the type of Data() is compatible with the return bool value.

[chapulina]

we don't immediately know the type of Data() is compatible with the return bool value

After casting the component to components::WindMode, yes, the data will always be bool. If WindMode's data type ever changes, this will fail compilation. Note that this is different from CanonicalLink above, which doesn't hold any data.

I guess it may be hard for a developer to know that the type of WindMode is bool without looking at that class' definition, but I'm not sure how to make this clearer here.

Is comp->Data() always going to be true if set, or can it be set as true or false?

Any system could change the set the value to true or false.

[claireyywang]

Are we doing one sentence per line?

[chapulina]

I usually try to keep each line to less than 80 chars so it's easier to see diffs in the future. Would you prefer one sentence per line? Some sentences are quite long...