make 'input:' optional in call body

[mlin]

Just scratching a holiday itch....the input: heading in call bodies is useless clutter AFAICT. Make it optional (for backwards compatibility).

Perhaps the idea was for call bodies to have additional sections (e.g. runtime: for workflow-level overrides). Those could still be added in the future, whilst keeping input: implied if not present.

Checklist

• [x] Pull request details were added to CHANGELOG.md

[rhpvorderman]

I like this idea. Less is more. I like how it is optional too, having maximal backwards-compatibility where possible is the only way forward. When WDL 2.0 eventually launches I don't want it to become the python 3.0 disaster where only the core devs are on the new stuff and the rest of the world waits for 10+ years.

[patmagee]

I love this idea too. I have always felt that in lieu of other properties it was just added boilerplate to the call definition. @mlin are you able to take a stab at updating the grammar bychance?

[jdidion]

@mlin please change target to wdl-1.2