timeseries/spectre (Fix issue 269)

[kkappler]

• migrates the multivariate tools that had been stashed in mth5/utils/ during initial development to timeseries/spectre.
• Renamed this module from fc_tools.py to multiple_station.py
• brings the Spectrogam class from aurora into mth5 (this is a fairly general class that can be used to wrangle FCs, and should not be aurora-specific)
• update imports accordingly

@kujaku11 the PR to bring this into features is an FYI to clarify these changes from what is already in patches. When you approve please consider merging patches into main as well to allow aurora to import these methods from mth5 moving forward.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 54.73684% with 43 lines in your changes missing coverage. Please review.

Project coverage is 62.47%. Comparing base (113f1b0) to head (2c131cc).

Files with missing lines	Patch %	Lines
mth5/timeseries/spectre/spectrogram.py	33.87%	41 Missing ⚠️
mth5/timeseries/spectre/multiple_station.py	80.00%	1 Missing ⚠️
tests/timeseries/spectre/test_spectrogram.py	93.33%	1 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           patches     #270      +/-   ##
===========================================
- Coverage    62.54%   62.47%   -0.07%     
===========================================
  Files          164      167       +3     
  Lines        17523    17606      +83     
===========================================
+ Hits         10959    11000      +41     
- Misses        6564     6606      +42     

Flag	Coverage Δ
tests	62.47% <54.73%> (-0.07%)	⬇️

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