add context cancelation

[pj]

Some of these algorithms take a long time to calculate for massive amounts of data. It would be nice if you implemented context cancelation.

[JIN Jie]

Could you please elaborate?
I'm not sure how context cancelation should be integrated into this lib, some example or a pull request would be nice.

[pj]

For each function that potentially takes a long time to return, add ctx context.Context as the first argument.

Then inside the function, check for if err := ctx.Err(); err != nil { return err }

[pj]

?