Rename SEDec to SDEC in Spectral element DEComposition plot

[jaladh-singhal]

Changed all SEDec to SDEC in module name and code.

Motivation and Context

SEDec still includes SED that stands for Spectral Energy Distribution which may cause confusion to some users. Thus SDEC is a better choice.

How Has This Been Tested?

• Testing pipeline
• Reference Data Comparison following these instructions
• Other (please describe)

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• None of the above (please describe)

Checklist:

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have built the documentation on my fork following these instructions
• I have assigned and requested two reviewers for this pull request

[codecov]

Codecov Report

Merging #1441 (1ea417d) into master (1053c90) will decrease coverage by 1.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1441      +/-   ##
==========================================
- Coverage   71.16%   70.03%   -1.14%     
==========================================
  Files          67       67              
  Lines        5521     5312     -209     
==========================================
- Hits         3929     3720     -209     
  Misses       1592     1592              

Impacted Files	Coverage Δ
...dis/tardis/montecarlo/montecarlo_numba/r_packet.py	23.15% <0.00%> (-7.83%)	⬇️
.../tardis/montecarlo/montecarlo_numba/interaction.py	24.32% <0.00%> (-5.68%)	⬇️
.../montecarlo/montecarlo_numba/single_packet_loop.py	24.56% <0.00%> (-3.78%)	⬇️
...rdis/tardis/montecarlo/montecarlo_numba/vpacket.py	15.00% <0.00%> (-2.48%)	⬇️
tardis/tardis/plasma/properties/util/macro_atom.py	30.00% <0.00%> (-2.26%)	⬇️
...rdis/montecarlo/montecarlo_numba/tests/conftest.py	91.11% <0.00%> (-1.88%)	⬇️
tardis/tardis/plasma/properties/nlte.py	39.21% <0.00%> (-1.74%)	⬇️
...s/tardis/montecarlo/montecarlo_numba/macro_atom.py	43.75% <0.00%> (-1.71%)	⬇️
tardis/tardis/montecarlo/spectrum.py	70.00% <0.00%> (-1.70%)	⬇️
...is/tardis/plasma/properties/continuum_processes.py	41.66% <0.00%> (-1.58%)	⬇️
... and 30 more

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[jamesgillanders]

looks good - happy for it to be merged