Add unified calibration class

[dbrakenhoff]

• add unified Calibration class that supports both steady and transient models
• add support for constants and time shifts for head observations
• ensure steady transmissivities are updated in initialize
• use inhomdict in steady models and support names
• improve aquifer summaries by using named columns instead of iloc indexing
• add print time info to tmin_warning in invlapcomp
• add example notebook to test new class

[dbrakenhoff]

This PR introduces a breaking change, the inhomlist has been replaced by an inhomdict in timflow.steady. I think the impact is small as most people weren't accessing the inhomogeneities that way... ?

[Copilot]

Pull request overview

This PR introduces a unified calibration framework intended to support both steady-state and transient timflow models (including shared/joint parameters), while also standardizing inhomogeneity handling (inhomdict) and improving aquifer summaries and some messaging.

Changes:

• Added timflow.calibrate.Calibrate as a new unified calibration API, and exported it from timflow.__init__.
• Replaced list-based inhomogeneity handling with inhomdict usage across steady/transient models and plotting, and updated summaries to use named columns.
• Improved transient inverse Laplace warning output and updated a benchmark notebook for the new inhomogeneity access pattern.

Reviewed changes

Copilot reviewed 11 out of 12 changed files in this pull request and generated 15 comments.

Show a summary per file

File	Description
timflow/transient/invlapnumba.py	Improves tmin warning printout by including offending time and interval info.
timflow/transient/inhom1d.py	Removes redundant initialize() override so element creation can be driven centrally.
timflow/transient/aquifer.py	Makes aquifer summary column handling more robust and ensures 2-pass inhom init/element creation.
timflow/steady/model.py	Updates inhomogeneity iteration and summaries to use inhomdict and inhom names.
timflow/steady/inhomogeneity1d.py	Adjusts naming initialization to rely on aquifer add_inhom() naming logic.
timflow/steady/inhomogeneity.py	Adds optional name support to polygon inhomogeneities and forwards it in constructors.
timflow/steady/aquifer.py	Recomputes transmissivity on initialize, switches to inhomdict, and improves summary indexing.
timflow/plots/plots.py	Unifies cross-section inhomogeneity plotting by iterating aq.inhomdict.values().
timflow/calibrate.py	Adds new unified calibration implementation (steady/transient, shared parameters, time shifts/constants).
timflow/init.py	Exports the new unified Calibrate at package top level.
docs/steady/04benchmarks/test_polygon_areasink.ipynb	Updates notebook to access inhomogeneity via inhomdict and adds an empty cell.

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