Refactor the functions Module

[andthum]

Refactor the functions Module

Type of change

• Change of core package.
• Change of scripts.

• Bug fix.
• New feature.
• Code refactoring.
• Dependency update.
• Documentation update.
• Maintenance.
• Other: Description.

• Non-breaking (backward-compatible) change.
• Breaking (non-backward-compatible) change.

Proposed changes

Breaking changes

• Rename the function mdtools.functions.g to mdtools.functions.line.
• Rename the function mdtools.functions.g_inverse to mdtools.functions.line_inv.

New features

• Function mdtools.functions.fit_exp_decay_log: Allow to parse keyword arguments to the underlying function scipy.optimize.curve_fit.
• Function mdtools.functions.fit_kww: Allow to parse keyword arguments to the underlying function scipy.optimize.curve_fit.

Documentation updates

• Update and format all docstrings and the source code in the module mdtools.functions.
• Add the module mdtools.functions to the official MDTools documentation.

PR checklist

• I followed the guidelines in the Developer's guide.
• New/changed code is properly tested.
• New/changed code is properly documented.
• The CI workflow is passing.