Add opencadd.io module

[dominiquesydow]

Description

Add an opencadd.io module with a first round of input/output functionalities. This module will undergo refactoring, once we decide on a more generalized architecture (how this will look like, still needs to be discussed, see this issue).

Todos

• Add io.dataframes (DataFrames from mol2 and pdb files/text) and io.rdkit (RDKit molecules from mol2 files/text) modules. Move code here from opencadd.databases.klifs.
• Add test data to opencadd.tests.data.io
• Add basic unit tests checking if output format is as excepted.
• Add tutorial showing the full API of this module.
• Add this module to the docs.

Questions

Status

• Ready to go