Add metadata attribute to spectral_data class

[k034b363]

Describe your changes
Previously, we had added some additional attributes to the spectral_data class to accommodate some of the metadata associated with geospatial data. This PR now replaces those individual metadata terms with an attribute called "metadata". The default is an empty dictionary, but it can be populated with any number of keys specific to various data types, like hyperspectral and geospatial.

Type of update
Is this a:

• New feature or feature enhancement

For the reviewer
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• PR functionality reviewed in a Jupyter Notebook
• All tests pass
• Test coverage remains 100%
• Documentation tested
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• Code reviewed
• PR approved

[deepsource-io]

Here's the code health analysis summary for commits 26bfd9a..a3ad6c3. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
Python	✅ Success		View Check ↗
Test coverage	✅ Success		View Check ↗

Code Coverage Report

Metric	Aggregate	Python
Branch Coverage	100%	100%
Composite Coverage	100%	100%
Line Coverage	100%	100%
New Branch Coverage	100%	100%
New Composite Coverage	100%	100%
New Line Coverage	100%, ✅ Above Threshold	100%, ✅ Above Threshold

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