Expand the README.md file

[livoire13]

• Make a distinction between required python packages and optional ones
• Installation using conda should use the conda-forge channel
• Make clearer what the software actually does and why it is useful (header of the readme file)
• In the 'Getting started' section, provide additional information on the tree-structure of the data directory and its sub-directories; explain how the user may set the FEMTOSCOPE_BASE_DIR environment variable when the working script and the data directory are not located in the same place
• Briefly comment the UML diagram: general data flow, mesh creation, how femtoscope builds on sfepy, the ifem technique, etc.
• In the 'Development mode' section, replace the name of the conda environment from femtoscope to something like femtoscope_env to avoid confusion