Add AM/PM/FM Demodulator & RSSI Estimator along with their testbenches

[fred-corp]

File description :

• .gitignore: added private and .DS_Store files
• source/impl_1/cordic_demod.vhd : modded CORDIC that works as expected (?)
• source/rx_chain/APFM_demodulator.vhd, tb_ng/apfm_demod/tb_APFM_demodulator.vhd & tb_ng/apfm_demod/run.py : AM/PM/FM Demodulator and its testbench
• source/rx_chain/RSSI_estimator.vhd, tb_ng/rssi_estimator/tb_RSSI_estimator.vhd & tb_ng/rssi_estimator/run.py : RSSI Estimator and its testbench

[fred-corp]

I know I make a lot of commits when I'm writing code; in my defence I do it to better divide tasks and TODOs :)