Use functions instead of methods for the interface functions

[ajfriend]

This PR:

• Define H3 API functions directly in their __init__.py (which is a shared identical symlinked file), using a relative import of _convert.py to distinguish the different APIs. This change should be invisible to end users.
• Uses lazy imports for numpy so that h3.api.numpy_int can be imported even if numpy isn't installed (but errors if you try to run a function that needs numpy)

After this PR:

• we'll do a follow up PR to cover Remove unordered outputs in v4 #335
• Combine coverage from symlinked API files #336

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (831d50b) 100.00% compared to head (f841381) 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##            master      #334    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           18        10     -8     
  Lines          513       324   -189     
==========================================
- Hits           513       324   -189     

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