Add unit-aware heat equation PDE

[chaoming0625]

This pull request includes several changes to the diffrax library to integrate unit-aware functionality and improve compatibility with the brainunit library. The most important changes include modifying the README.md to reflect the new unit-aware features, updating various files to import brainunit, and refactoring code to use brainunit's unit-aware functions.

Updates to documentation:

• README.md: Updated the project name to DiffraxU, changed the installation instructions, and modified the example code to use brainunit. [1] [2]

Codebase updates for unit-aware functionality:

• diffrax/__init__.py: Set the version to 0.6.1 and added compatibility with equinox using brainunit.
• diffrax/_brownian/tree.py, diffrax/_custom_types.py, diffrax/_integrate.py: Imported brainunit and refactored various mathematical operations to use brainunit's unit-aware functions. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]

Overall, these changes enhance the diffrax library by incorporating unit-aware capabilities, making it more robust and versatile for numerical differential equation solving.