README: install the julia interface through juliacall

Describe the juliacall setup in the manual installation (automatic julia
download, how to use an existing julia). PyJulia and PyCall are no longer
needed, also by tdscha (from version 1.6.2).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
(cherry picked from commit ae9865a)

Author: mesonepigreco

README.md

@@ -107,42 +107,31 @@ To activate the environment and execute the SSCHA, run
 The SSCHA benefits from Julia being installed in the system. If present,
 it will be automatically used to speedup the calculation.
 
-To install Julia, refer to the official website [julialang.org/downloads/](https://julialang.org/downloads/)
-Alternatively, to install Julia on Linux we can employ JuliaUp:
-
-```
-  curl -fsSL https://install.julialang.org | sh
-```
-
-Hit enter when asked to install Julia.
-
-Then, install the python bindings for julia with
+The python-julia interface is provided by [juliacall](https://github.com/JuliaPy/PythonCall.jl)
+(PyJulia is no longer required). Install it with
 
 ```
    pip install juliacall
 ```
 
-The tdscha extension to compute Raman and IR requires some additional julia packages that can be installed within a julia terminal. Update your configuration to have access to the newly installed julia
+There is no need to install Julia manually: the first time it is needed, juliacall downloads
+Julia and the required Julia packages automatically. You can trigger (and check) this step with
 
 ```
-  source ~/.bashrc
-```
-
-Then, open a terminal and type `julia`. Inside the julia prompt, type `]`. The prompt should change color and display the julia version ending with `pkg>`
-
-Install the required julia libraries
-
-```
-  pkg> add SparseArrays, LinearAlgebra, InteractiveUtils, PyCall
+python -c 'import sscha.JuliaExt as J; J.get_main()'
 ```
 
-This should install the required libraries. Press backspace to return to the standard Julia prompt and exit with
+If you prefer to use a Julia already installed in the system (e.g. from [julialang.org/downloads/](https://julialang.org/downloads/)
+or JuliaUp), point juliacall to the Julia executable (not the JuliaUp launcher) and to a Julia environment:
 
 ```
-  julia> exit()
+export PYTHON_JULIACALL_EXE=/path/to/julia/bin/julia
+export PYTHON_JULIACALL_PROJECT=/path/to/a/julia/environment
 ```
 
-Now, you should be able to exploit the Julia speedup in the TDSCHA calculations. It is not required to install Julia before TDSCHA; it can also be done at a later time.
+The tdscha extension to compute Raman and IR (from version 1.6.2) uses the same juliacall interface,
+and the required Julia packages are installed automatically: no further setup is needed.
+It is not required to install Julia before TDSCHA; it can also be done at a later time.
 
 
 ### Compiling SSCHA