Support optional fit and produce args and arg defaults

[csala]

The current parsing of fit and produce args requires all the arguments to always exist inside the pipeline context.

Two new keywords should be added inside the arg specification dictionary to make this behavior more flexible:

• default: a default value to pass to the method if the variable is not found in the pipeline context.
• required: if set to False and the variable is not found in the pipeline context, do not pass it to the method. This will be True by default.