DataFrame.rename() mapper arg should be typed instead of any #345

In version 1.0.0, the DataFrame.rename() mapper argument is typed as any instead of strongly typed. This caused me to completely miss the fact that the mapper argument is now outside the options argument while migrating to this new version of Danfojs, and it may lead others to do so as well. Wouldn't it be better to type it something like this?

type Mapper = {
    [index: string]: string | number;
}