feat: add opt-in literal output layout

[viranovskaya]

Closes #142.

Why

MEEGqc currently mirrors a BIDS-style tree under an external output directory. That remains useful as the default, but it adds dataset and derivatives levels when a caller needs the selected directory to be used literally.

What changed

This adds a named output_layout option while keeping the current BIDS-style layout as the default.

• one dataset in literal mode: <output>/MEEGqc
• multiple datasets in literal mode: <output>/<dataset-name>/MEEGqc
• calculation, plotting, and GQI use the same path resolver
• existing positional Python calls keep their parameter order
• the CLI and Python API are documented

Verification

The full unit, GUI, and official real-data fixture suite passes: 53 tests. A real-data end-to-end test also runs calculation and plotting in literal mode and checks the generated report.