This push request add the ligand/receptor data to the package and uses package data within functions

[Xiaojieqiu]

Firstly, I added a __init__.py file in the spateo/tools/database (where the ligand-receptor files are stored)

Then I added a MANIFEST.in file, understand the same folder of setup.py, with the following text:
recursive-include spateo/tools/database *

Then the setup.py file included the line of
include_package_data=True,

We use the following code to access the package data:
path = importlib.resources.path("spateo", "data") if path is None else path

Read more here and here, especially the second answer

Code in the cell_communication.py are better formated and missing documentation of args are included

[Xiaojieqiu]

@luluZuo can we have these files:

ligand_target_matrix and ligand_target_matrix_mouse to the database folder? Is it too big?