Deprecated toJSON, toCSV, and toExcel methods in frame and series

[risenW]

Closes #439

This PR deprecates toJSON, toCSV, and toExcel methods. That means we can no longer call them directly on DataFrames and Series, we instead favor using the functions directly. For example:

const dfd = require("danfojs")

let data = {
    Abs: [20.2, 30, 47.3],
    Count: [34, 4, 5],
    "country code": ["NG", "FR", "GH"],
};

let df = new dfd.DataFrame(data);

df.toCSV({ filePath: "testOut.csv"}); // 👎🏾   won't work anymore from v1.1.0

dfd.toCSV(df, { filePath: "testOut.csv"}); // 👍🏾  Prefered and will work

WHY DID WE DEPRECATE THESE METHODS?
The major reason for deprecating these methods In favor of the functional style is because it is easier and cleaner to separate browser implementation from Nodejs implementation. This ensures we don't do more weird bundling and support Webpack >5 out of the box without extensive configuration.

[idhamari]

In my case, nothing is saved and no error message. Any thought? I am using windows and CDN link for the library

[risenW]

In my case, nothing is saved and no error message. Any thought? I am using windows and CDN link for the library

Seen this: https://danfo.jsdata.org/api-reference/input-output/danfo.to_csv#convert-dataframe-to-csv-string-and-download-file-in-client-side-lib