Add BMI 2.0 interface and Python bindings

[HendrikKok]

Overview

This PR adds a Basic Model Interface (BMI) 2.0 implementation for Daisy,
along with Python bindings that allow Daisy to be driven from Python
using the BMI standard.

Changes

C++ BMI interface

• Add bmi.h / bmi.C: BMI 2.0 base interface
• Add daisy_bmi.h / daisy_bmi.C: Daisy-specific BMI implementation
  wrapping the core Daisy and Column objects
• Expose key variables (soil water, groundwater) via BMI get/set methods
• Add DaisyPythonController for driving Daisy from Python

Python package (python/)

• New daisy Python package with BMI bindings (bmi_bindings.cpp)
• Moved DaisyPyFun plugin interface into the new package
• Example scripts:
  1- daisy_bmi_example.py for a bmi driven simulation,
  2- updated daisy_py_fun_example.py to use the new python package
• Updated pyproject.toml

Build

• Updated CMakeLists.txt and build scripts for the new targets
• Added sample_bmi.dai sample setup file

Status

• [ x] Windows build tested
• Linux build tested
• More BMI variables to expose (work in progress)

Notes

• This is a draft — feedback on the interface design and which variables
  to expose via BMI is welcome.
• build.bat / build_exe.bat are included as a convenience for Windows builds
  and will be replaced by proper README documentation in a follow-up.

BMI state access: towards a central MemoryManager

The current BMI implementation pierces the full model hierarchy
(BMI → DaisyBMI → Daisy → Field → Column → SoilWater) with a separate
getter at every layer for each variable. Every new BMI variable requires
adding that method at every level, and every read copies a std::vector.

The root cause is that SoilWater and other components have private variables,
so every caller must go through a getter to reach them.

Proposed direction: global MemoryManager

Models register live pointers to their internal arrays once at
initialize() time. BMI reads from the manager by name — no per-variable
forwarding chain, no copies.

// SoilWater::initialize() registers its arrays:
store.register_view("soil_water__pressure_head", h_.data(),        h_.size());
store.register_view("soil_water__theta",         Theta_.data(),    Theta_.size());
store.register_view("soil_water__flux",          q_matrix_.data(), q_matrix_.size());

// BMI::get_value_ptr — zero copy, standard BMI 2.0:
void* BMI::get_value_ptr(const std::string& name) {
    return const_cast<double*>(memory_manager_.get(name).data);
}

// BMI::get_value — copies from the same pointer:
auto view = memory_manager_.get(name);
std::copy(view.data, view.data + view.size, dest);