Increase package modularity

[davide-f]

The package needs restyling to improve modularity and maintenability.
In particular, block.py is way too large and shall be disentangled in:

• config.py dedicated to configuration, hence SMSConfig
• plot.py for plotting
• io.py for input/output
• definitions.py for Block, Variable, Dimension and Attribute definitions
• standard_blocks.py [or equivalent] to read the standard block types
• tools.py for smspp tools. Ideally, the solver registry shall be generalized and extended

modules may be created