v7.1

Major changes:

• introduce Jupyter Notebooks section (@carlos-encs)
• document Python virtual environments more in detail (@carlos-encs)
• make sure --mem usage is mandatory (@carlos-encs)
• replace $USER usage in #SBATCH comments (@carlos-encs)
• improve clarity of language (@yulgroper. @smokhov)
• update the list of sister facilities (@smokhov)
• add a multi-CPU training example for PyTorch (@carlos-encs)
• general fixes (@smokhov, @yulgroper)