Installation - eigen version issue

Hi Antonio,

I encountered a small error when installing amber. It's because Eigen 5.0.1 was installed by default.
```
CMake Error at CMakeLists.txt:163 (find_package):
  Could not find a configuration file for package "Eigen3" that is compatible
  with requested version "3.3"
```

I fixed the error with:
```
conda remove eigen
conda install -c conda-forge "eigen=3.4"
```

Best,
Cliff