Fix wavespeed estimates for SWE-Exner equations

[patrickersing]

This fixes a bug where max_abs_speed and max_abs_speeds_naive didn't take absolute values into account.
To my surprise, none of the test values need to be updated as the situation with a dominant negative eigenvalue doesn't seem to occur in any of the tests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.27%. Comparing base (846f4a1) to head (745be79).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #55   +/-   ##
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  Coverage   99.27%   99.27%           
=======================================
  Files          61       61           
  Lines        2630     2630           
=======================================
  Hits         2611     2611           
  Misses         19       19           

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[andrewwinters5000]

LGTM!