Fix import of inner count min for TopK import

[JustroX]

Hello, thanks for creating this library! It's been very helpful.

I'm currently working with the TopK data structure and encountered a bug.
It seems that it's throwing this error whenever I am trying to update an imported TopK.

TypeError: this._sketch.update is not a function

This is the code for the bug replication
https://stackblitz.com/edit/node-kcv3z7?file=index.js

// Initialize topK
const topk = new TopK(k, error_rate, accuracy);
topk.add('Alice');
topk.add('Bob');

// Save topk
const json = topk.saveAsJSON();
const str = JSON.stringify(json);

// Import topK
const new_topk = TopK.fromJSON(JSON.parse(str));

// Update topk
new_topk.add('Carl');  // this throws

I figured that this might be due to the inner count-min sketch not being properly initialized during import.

I am not that confident that I understand what the decorators do, but based on how the inner heap is imported I made the following fix in the TopK class.

@Field<CountMinSketch>(
  (sketch: CountMinSketch) => sketch.saveAsJSON(),
  (json: any) => CountMinSketch.fromJSON(json)
)
private _sketch: CountMinSketch

I also made the corresponding tests for this. I hope this fixes the bug.