Create fetchChemicals method

Currently we don't have a method that fetches the list of chemical ids by the 'type of id (ex. CasRN, DTXSID).'
The goal is to create a method called fetchChemicals which takes input parameter of 'type of id (ex. CasRN, DTXSID)' and 'list of ids' to return a list of chemicals matching those id list input.

TODO:
- Create method fetchChemicals