Refactor pspm_pipeline_fc_scr API and improve usability

[gjheij]

Fixes #896.

Switched to name–value pair arguments

Replaced positional optional arguments with inputParser
Supported options:

• 'method' (default: '2026_long_uni')
• 'missing' (default: [])
• 'normalize' (default: 0)
• 'keepfile' (default: 0)
• 'overwrite' (default: 0)

Simplified function signature

pspm_pipeline_fc_scr(fn, onsets, isi, varargin)

Improved input consistency

• All inputs internally normalized to cell arrays
• Explicit checks for session alignment
• Robust handling of scalar vs vector ISI

Cleaned up model file handling

• Safe generation of model_fn for single and multiple files
• Avoids fragile assumptions about fileparts output types

Also made updates to documentation.