Avoid connecting to FTP server on import

[fccoelho]

When one imports the SINAN module, the line below makes a connection to the FTP server.

PySUS/pysus/online_data/SINAN.py

Line 8 in fda2a29

sinan = SINAN().load()

In situations where the server may be unavailable, this will raise an exception. This connection should be established only when needed and within a try/except clause to avoid crashing code using PySUS as a library.

[YuF-9468]

I'd like to take this one. I'll work on a small fix to avoid the eager FTP connection during pysus.online_data.SINAN import and add a regression test.

[YuF-9468]

I investigated this and opened a low-risk fix PR:
#243

It makes pysus.online_data.SINAN import lazy so it doesn't open an FTP connection during import.