add NRZI encoding decoding

[ErikBuer]

Hi professor Gerzaguet, hope everything is well.

I have added NRZI encoding decoding with some explanatory tests.
First, I thought the functions were suitable for the file bitMapping, as it is a mapping to the NRZI format.
But the file is in the documentation under something QAM-related, so I made a separate file for them.

I have also added some quality-of-life enhancements for building docs locally.
This will make it faster to build docs locally, and should not affect the docs build in CI. I have done similarly in my package radioPropagation.

In the documentation I see you have used the term "bit sequence" and "byte sequence". In other sources, I see people using the terms packed bits and unpacked bits, bit array and byte array.
I feel we should make a small write-up (example in the docs) to explain/visualize the terms and what they mean. Using the various terms will hopefully help the library appear in searches, helping others on their journey.
This must be in a separate PR, but I thought I'd mention it..
I have adopted the term bit sequence, to stay consistent in this library.

[RGerzaguet]

Ok for this !

[RGerzaguet]

Many thanks Eric for this very complete and useful add of NRZI !
Few remarks (I have updated the code except the last point)

• While the API is great (and consistent with QAM modulation for instance) I think that we should also add bang methods (!) with pre-allocation
• The code may be a little optimized in terms of pre-allocation to be able to run at full speed. While this is not a key element for classic simulations, it may help when it comes to high performance needs. As we now the size of the output this is not that tricky to code I believe.
• Last point let's add some unit tests :)
  On your side, may you add a test_NRZI.jl file in test/ with something like

# ---------------------------------------------------- 
# --- Import modules  
# ---------------------------------------------------- 
using DigitalComm 
using Test
# ---------------------------------------------------- 
# --- Tests 
# ---------------------------------------------------- 
# Note --> The mapping system is described in bitMapping.jl
println("Tests for symbol mapper with NRZI sequences");

@testset  "NRZI" begin 
	# Create a bit squence (already tested) 
	nbBits	= 2 * 2048;
	bitSeq  = genBitSequence(nbBits);
	# Pass trough the function 
	buff	= zeros(Complex{Float64},nbBits)
	# Call 
	encodeNRZI!(buff,bitSeq); 
	buff2	= encodeNRZI(bitSeq);
	@test all( buff .== buff2)
        # Ensure Tx // Rx is Ok 
        @test all(bitSeq .== decodeNRZI(encodeNRZI(bitSeq)))
        @test all(bitSeq .== decodeNRZI(encodeNRZI(bitSeq,:high),:high))
	# Some manual check for both transitions
	buff  = [0x01;0x00;0x00;0x01;0x00;0x00;0x00;0x01 ];
        @test all( encodeNRZI(buff,:low) .== [0;1;0;0;1;0;1;1])  # Transitions on 0 
        @test all( encodeNRZI(buff,:high) .== [1;1;1;0;0;0;0;1]) # Transitions on 1
end

and the associated include in the core test suite (runtests.jl) line 19

# NRZI Mapping 
include("test_NRZI.jl");

Thank you very much for keeping DigitalComm such active and for contributing, I appreciate it a lot.

[RGerzaguet]

Re @ErikBuer
If this is Ok for you, I will merge this PR and create a new version for DigitalComm.
If you want to update the code further, do not hesitate.
Thanks again !