Refactor api submodule

A subfolder with a single submodule (zenodo.py) in where all functions are imported to the init.py of the parent submodule (api) is redundant. Refactoring to avoid misleading imports.

This will make imports

from eoss.api.eossr import <name>
from eoss.api.zenodo import <name>

eoosr
├── api/
|   ├── zenodo/
|   |   ├── __init__.py
|   |   └── zenodo.py
|   ├── __init__.py
|   ├── ossr.py
|   └── http_statys.py

to

eoosr
├── api/
|   ├── __init__.py
|   ├── zenodo.py
|   ├── ossr.py
|   └── http_statys.py