Easy slot access

Allow simplified i/o slot access via component's __getitem__(name) operator.

Default access:

comp_a.outputs["Out_1"] >> comp_b.inputs["In_1"]

With this feature:

comp_a["Out_1"] >> comp_b["In_1"]

• Implement it
• Test it
• Document in API
• Document in book

Fixes #71 (closed)