Fix logic of @import

[kris-brown]

Addresses #167

This changes what @import does within the @instance macro. Rather than removing expected signatures, whenever we have leftover unimplemented signatures we check if the name of the method is in the imports and ignore the missing method error if so.

I've tested that this doesn't break Catlab.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.56%. Comparing base (a129cb4) to head (1b42e18).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #168      +/-   ##
==========================================
+ Coverage   93.85%   95.56%   +1.70%     
==========================================
  Files          38       38              
  Lines        2229     2231       +2     
==========================================
+ Hits         2092     2132      +40     
+ Misses        137       99      -38     

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