Some attributes of Session are missing

Restructuring here:

https://github.com/pyansys/pyfluent/pull/691/files#diff-bd3f73b4605be1cd370054c76099892c1cdc3e5ce68fd99167796db8cb194a3e

puts some attributes, such as field_data, in the fluent_connection object without being referenced in the session objects. I have the necessary fix in my local changes.