Fix ShallowWaterExnerEquations1D for negative / zero velocities

[patrickersing]

This PR fixes some bugs that occur when ShallowWaterExnerEquations1D are applied for test cases with negative or zero velocities. Theses errors were not covered by the current test setup, so I also added a new elixir elixir_shallowwater_exner_dam_break_symmetric to cover these cases.

There were two bugs present:

• To avoid division by zero when computing the effective sediment height h_s we introduce a condition v<eps() to introduce a special treatment for zero velocities. This was changed to abs(v) < eps() to not apply this for negative velocities.
• The computation of h_s_avg in the dissipation_roe was changed to avoid division by zero velocities.

In addition to that I also updated the representation of some constants to be compatible with single precision and introduced a type conversion to the constructor of the sediment models.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.19%. Comparing base (75de71d) to head (c1bf10e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #101   +/-   ##
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  Coverage   99.19%   99.19%           
=======================================
  Files          72       73    +1     
  Lines        3449     3459   +10     
=======================================
+ Hits         3421     3431   +10     
  Misses         28       28           

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[andrewwinters5000]

LGTM!