20230205 add schema check logic on dbloader constructor

[jorgeantonio21]

Adds logic to execute a simple create schema on DB, if the passed schema_name does not exist. Also replaces &String with &str in the signature of methods.

Warning: very rough, it checks directly on the DB if schema exists.

[jorgeantonio21]

Resolves #48

[jorgeantonio21]

See #67 for further discussion and motivation for current PR.