Flush stdout before each call to _process_chunk

[joezuntz]

Problem & Solution Description (including issue #)

When I redirect BPZ's catalog estimator to file on MacOS I don't get any logging output until right at the end due to buffering. This change adds a flush so that the user can see progress even it it is logged to file. It should have no other effects.

Code Quality

• My code follows the code style of this project
• I have written unit tests or justified all instances of #pragma: no cover; in the case of a bugfix, a new test that breaks as a result of the bug has been added
• My code contains relevant comments and necessary documentation for future maintainers; the change is reflected in applicable demos/tutorials (with output cleared!) and added/updated docstrings use the NumPy docstring format
• Any breaking changes, described above, are accompanied by backwards compatibility and deprecation warnings

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.75%. Comparing base (102b27a) to head (1b0d44c).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #207   +/-   ##
=======================================
  Coverage   99.75%   99.75%           
=======================================
  Files          47       47           
  Lines        3317     3319    +2     
=======================================
+ Hits         3309     3311    +2     
  Misses          8        8           

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