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

[tpoisseau]

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);