Function to extract fink classification from the alert doesn't need pyspark

[Theodlz]

In
/fink_filters/classification.py pyspark is imported on top of the file. However, for most functions defined there (2 out of 3), pyspark is not needed nor used. Only extract_fink_classification uses it, but extract_fink_classification_from_pdf(pdf) doesn't.

What's the expected behavior :

We would expect pyspark not to be needed when a user imports extract_fink_classification_from_pdf(pdf) to extract the classification from a simple panda dataframe, or even when using extract_fink_classification_() that just needs the right fields from the alert (this function is used by the one mentionned before, and the one mentionned next). But only when using extract_fink_classification that is meant to be used with Apache Spark.

What can be done :

The easiest solution would be to have the basic classification function extract_fink_classification_ (used by both panda and spark functions later on) in its own module. And then 2 seperate modules, one that imports pandas as needed, and one that imports spark, where both import the module that contains extract_fink_classification_.

Otherwise, maybe just finding a way to import the right libraries used by each function, rather than all of them when importing the whole module (what i mean is that, if i import only ``extract_fink_classification_from_pdf, it could maybe only import the libraries used by it, and the same would happen when importing only extract_fink_classification` where this time it would import pyspark).

[JulienPeloton]

Priority when I return from holidays ;-)