Script to create Seldon API testing files from any Pandas dataframe

[Love-R]

This PR adds functionality to create Seldon API testing files from any Pandas dataframe. The work has been tested with seldon-core-tester on private data.

I am not sure where this functionality belongs to, please point me a relavant folder if you want it elsewhere.

Details:

• Ftype and dtype in the output file is based on dtype of each column of pandas dataframe. All numbers are treated as continuous and non-numbers are categoricals.
• For continuous variables, the range for testing is based on minimum and maximum values in the file. If all values of a variable are missing, the default range is used.
• For categorical variables, values are all unique values in the dataset.

This is a joined work with Oleg Shevelev.

[Love-R]

@cliveseldon What is the procedure to get this merged?

[ukclivecox]

This looks good. Happy to merge to master. Be good to add some docs in https://docs.seldon.io/projects/seldon-core/en/latest/workflow/api-testing.html