Refactor and add ADC model capability

[fronzbot]

• Refactored code a bit
• Added skeleton to start including ADC models
• Added basic CIC and FIR decimation filters
• Modified outputs of spectral functions a bit
• Working on improving docstrings

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 6.51163% with 201 lines in your changes missing coverage. Please review.

Project coverage is 31.96%. Comparing base (9055b27) to head (092c614).

Files with missing lines	Patch %	Lines
adc_eval/filter.py	0.00%	100 Missing ⚠️
adc_eval/adcs/basic.py	0.00%	69 Missing ⚠️
adc_eval/eval/spectrum.py	33.33%	28 Missing ⚠️
adc_eval/signals.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              dev     #104       +/-   ##
===========================================
- Coverage   66.47%   31.96%   -34.51%     
===========================================
  Files           4        4               
  Lines         173      341      +168     
===========================================
- Hits          115      109        -6     
- Misses         58      232      +174     

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