initial commit
Refactoring global variables and initialization

Added some comments describing the variables, and what they are used
for.
Added a slider to adjust the SPL threshold live

I added a Qt slider, and linked the signal `valueChanged` to the variable
SPL_THRESHOLD which fixes the frequencies to be displayed.
blob with continuous line
Blob with lines and not continuous curves
improved sidebar appearance
modified gitignore
before gui split
added some tests
refactoring with new directory structure and split modules