Refactor _Advanced Usage_ section in the doc and correct errors therein

[lelaus]

This issue is related to the tasks of Advanced Usage section in #147.

• put the paragraph related to the available_correlations() example outside the bulleted list;
• transform the two bullets into two subsections;
• typo: from "Let" to "Let's"
• pass separator in different OS (already treated in Checking separator when calling set_custom_properties_path() method #153)
• focus on listing only the relevant correlations, that is the ones available for rho
• modify Lead.correlations_available() into Lead.available_correlations()
• "If the density correlation is not specified for a new object instantiation, the last one in the list will be selected as default:": currently, the example below shows only what happens by using the Lead.set_correlation_to_use(); an example should be added showing the default behavior as noted here if the function Lead.set_correlation_to_use() is not used
• modify the path <execution_dir>/custom_property into <execution_dir>/custom_properties.

[lelaus]

Task n.6 treated in #144.

[lelaus]

Task n. 1 handled in 1e77498.

[lelaus]

Task n.2 handled in 31c3cca.

[lelaus]

Task n.5 handled in 7454d24.

[lelaus]

Task n.8 handled in c133269.

[lelaus]

Task n.7 handled in 341a185.

[panDanieleN]

Merged into master.

Updated doc as described.
No need to update tests.