Move to src layout

[gilrrei]

Hello QUEENS people,

we moved QUEENS from a flat to src layout in #174. This means instead of:

├── config
├── doc
├── queens
│   ├── data_processors
│   ├── distributions
│   ├── drivers
│   ├── example_simulator_functions
│   ├── iterators
│   ├── models
│   ├── parameters
│   ├── schedulers
│   ├── stochastic_optimizers
│   ├── utils
│   ├── variational_distributions
│   └── visualization
├── templates
├── tests
└── test_utils

QUEENS now looks like this

├── config
├── doc
├── src
│   ├── queens
│   │   ├── data_processors
│   │   ├── distributions
│   │   ├── drivers
│   │   ├── example_simulator_functions
│   │   ├── iterators
│   │   ├── models
│   │   ├── parameters
│   │   ├── schedulers
│   │   ├── stochastic_optimizers
│   │   ├── utils
│   │   ├── variational_distributions
│   │   └── visualization
├── templates
├── tests
└── test_utils

This is a future-proofing step. The source layout is more common in Python projects and simplifies the package setup. In the future, additional packages will emerge, see #176.

The change is backward compatible but it is recommended to reinstall QUEENS via

pip --no-deps -e .

If you encounter any problem let us know.

[sbrandstaeter]

We talked about this for a long time and finally made it thanks to your (@gilrrei) initiative.