Extend grounder API

[bgyori]

This PR adds the following API functions to expose more functionalities:

• get_grounder to return the default grounder instance (which was previously only available through the API in a convoluted way)
• make_grounder and entrypoint to create a custom grounder directly via the main API (previously one had to import the Grounder class and initialize it to do this)
• get_ambiguities return ambiguities detected in a grounder instance
• the constructor of the Grounder class as well as make_grounder now also take lists of Terms as input which is more convenient