Procedure return interrupts

[newville]

This fixes #146, a problem introduced when fixing #141 with #142. In that fix, we flagged "return" statements as interrrupts to a while/for/etc block.

But, when running a asteval-defined function/procedure, a return statement must not interrupt calling code - say, calling the procedure in a block.

This fix simply unsets asteval._interrupt - the flag that a loop-interrupt has happened - when a procedure/function returns.

Added a test based on example at #146. Thanks @sergproua!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.4%. Comparing base (bda05fd) to head (1984dbf).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #147   +/-   ##
======================================
  Coverage    95.3%   95.4%           
======================================
  Files           4       4           
  Lines        2171    2182   +11     
======================================
+ Hits         2070    2082   +12     
+ Misses        101     100    -1     

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