Add two helper functions for calculating bandwidth

[simonrp84]

The bandwidth of a satellite sensor is the wavelength range over which it is sensitive to a particular level of signal. This is typically defined in one of two ways:

1. The full width half maximum: The total range of wavelengths over which the spectral response function is greater than half of the maximum response.
2. The integrated energy response: The minimum and maximum wavelengths over which a sensor is sensitive to more than a given threshold of the integrated energy measured by the sensor.

This PR adds utility functions to compute these two parameters. It's still a draft as I need to add documentation and tests.

• Closes #xxxx
• Tests added
• Tests passed: Passes pytest pyspectral
• Passes flake8 pyspectral
• Fully documented
• Add your name to AUTHORS.md if not there already

[codecov]

Codecov Report

❌ Patch coverage is 15.38462% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.90%. Comparing base (c1283c6) to head (1c67287).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
pyspectral/utils.py	15.38%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #259      +/-   ##
==========================================
- Coverage   90.28%   89.90%   -0.39%     
==========================================
  Files          22       22              
  Lines        2523     2536      +13     
==========================================
+ Hits         2278     2280       +2     
- Misses        245      256      +11     

Flag	Coverage Δ
unittests	89.90% <15.38%> (-0.39%)	⬇️

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