examples: Refactor wavelets

[JDBetteridge]

In response to being told to create the examples.RickerSource object, whenever I redefined it as a function.

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Codecov Report

:x: Patch coverage is 35.21127% with 46 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 53.24%. Comparing base (3169bb5) to head (6751e07).

Files with missing lines	Patch %	Lines
examples/seismic/source.py	35.21%	39 Missing and 7 partials :warning:

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HEAD has 10 uploads less than BASE

Flag	BASE (3169bb5)	HEAD (6751e07)
	18	8

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2770       +/-   ##
===========================================
- Coverage   83.04%   53.24%   -29.81%     
===========================================
  Files         248      248               
  Lines       50469    50511       +42     
  Branches     4440     4452       +12     
===========================================
- Hits        41913    26895    -15018     
- Misses       7795    22640    +14845     
- Partials      761      976      +215     

Flag	Coverage Δ
pytest-gpu-aomp-amdgpuX	68.67% <35.21%> (-0.06%)	:arrow_down:
pytest-gpu-nvc-nvidiaX	69.21% <35.21%> (-0.05%)	:arrow_down:

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[mloubout]

Also see ##2743, there is a tiny discretization issue that makes the discrete wavelet non-zero mean, which can be problematic.