mf-parser: `MF.prototype.getInfo` can be incoherent on successive calls due to caching

```ts
const mf = new MF('Na+.Cl-');

// OK
expect(mf.getInfo().monoisotopicMass).toBe(57.958621964);

// error, undefined, because getInfo is cached in the instance
expect(mf.getInfo({emFieldName: 'test'}).test).toBe(57.958621964);
```

Start with MF.prototype.getInfo and reproduce the successive-call example from the issue. Trace how the instance cache handles the emFieldName option; done means both calls return the expected mass and the behavior is covered by a regression test.

Written by the indexing model from the issue text.