Horizontal relationship exploration

**Is your feature request related to a problem? Please describe.**
getCandidateCodes is fantastic at finding codes from different strings, but some vocabularies like SNOMED have very powerful horizontal relationships that could help with more complex phenotypes or with finding diseases based on symptoms.

**Describe the solution you'd like**
Add an option to search using concept relationships, or have a separate function for it. Add a function to get all potential types of relationships of a code. Happy to discuss further and help develop!