1. nrLDPC_coding_segment_encoding: Clarifying encoder parameters
   The way the encoder implementation params was used could be made clearer:
   * `macro_num` was always used in order to calculate segment indexes.
     It is clearer to pass directly the first segment index.
   * The initialisation of a common implementation parameters structure can be made clearer.

2. nrLDPC_coding_segment_encoder: Remove multi segment dimension in encoder output
   The output of `LDPCencoder` was a 2D unsigned char array.
   But all the segments were actually packed in the first segment, 8 bits of different segments per byte of the first segment.
   So the first dimension was useless. This commit removes this dimension.

3. nrLDPC_coding_segment_encoder: Rename `nrLDPC_prepare_TB_encoding` into `nrLDPC_launch_TB_encoding`

4. nrLDPC_coding_segment_encoder: Remove the useless second rate matching for E2, one rate matching with max(E, E2) does the job.

5. nr_interleaving_ldpc: cleanup
   Rewrite code to use different SIMD sizes in a linear fashion.
   This means cascading preprocessor directives are used to start from a high SIMD size an decrease it progressively.

6. ldpc_encoder_optim8segmulti: linear usage of SIMD
   Use cascading preprocessor directives to include different SIMD sizes and types depending on which are available on the system.
   The different loops use a common index that is progressively incremented.
   Also add AVX512 version of bits distribution from 8 segments into one byte.
Co-authored-by: Laurent Thomas <laurent.thomas@open-cells.com>