docs: update installation guide to use pixi package manager (#120)

Author: melund

docs/install_guide.md

@@ -4,33 +4,60 @@
 
 ### The easy way
 
-The easiest way to install AnyPyTools on Windows (along with all dependencies) is through the Anaconda Python
-Distribution and the conda package manager.
+The esiest way to get started is using the pixi package manager. It will
+automatically install python and all needed depencies for you. 
 
-Install AnyPyTools with the following command:
 
-```bat
-> conda config --add channels conda-forge
-> conda install anypytools
+Install the [pixi](https://pixi.sh/) package manager for Python.
+
+```bash
+powershell -ExecutionPolicy ByPass -c "irm -useb https://pixi.sh/install.ps1 | ie
 ```
 
-This will install `anypytools` and all the recommended dependencies. Next, try to launch the
-interactive AnyPyTools notebook tutorial :
+After installation, open your working directory for you project in the terminal
+and run the following command:
 
-```bat
-> AnyPyToolsTutorial.bat
+```bash
+pixi init
+pixi add anypytools
+pixi install
 ```
 
-### Other installations options
+This will create a virtual python environment in that folder and install the
+AnyPyTools package. Likwise, you can add other depedencies to the virtual
+environment (e.g. `pixi add jupyter pandas`). All dependencies are tracked in
+the `pixi.yaml` file which can also be edited manually. 
+
+You can now activate the virtual environment with `pixi shell` and run your
+scripts. Otherwise, you can prefix your commands with `pixi run` to run them in
+the virtual environment, e.g. `pixi run python myscript.py`.
+
+
+### Controling the version of AnyBody used by AnyPyTools
 
-It is also possible to install directly from the python package index. 
+The clever part of using a package manger like pixi is that you can also control
+which version of the AnyBody is used by AnyPyTools.
 
-```bat
-> pip install AnyPyTools 
+A light gui-less version of AnyBody exists as
+[conda-packages](https://anaconda.org/anybody/anybodycon). Hence, we can install
+a specific version of AnyBody into our virtual environment. 
+
+To do this we first need to add the `anybody` channel to our project. 
+
+```bash
+pixi project channel add anybody` 
 ```
 
-or clone/download the source files from [GitHub](https://github.com/AnyBody-Research-Group/AnyPyTools), and then run the following command in the source folder to install: 
+Then we can install a specific version of AnyBody console application. 
 
-```bat
-> pip install -e .
+```bash
+pixi add anybodycon=8.0.4
 ```
+
+Now all simulations executed with AnyPyTools in this project will use the
+specified version of AnyBody, regardsless of what is installed on the host
+system.
+
+:::{note}
+You still need valid AnyBody license to run the simulations with the AnyBody Conda packages. 
+:::