`nr_generate_pdsch()` clean up and little performance improves

• Improve PDSCH layer mapping by 32 bits assignment

• Improve PDSCH layer precoding by cleaning up C code, no new algorithm introduced.

• Use c16_t instead of casting int16_t* / int32_t* everywhere (in nr_generate_pdsch() and init_codebook_gNB())

• Cleaning up: Use macros instead of hardcoded numbers, Use shortcuts for array offset.

• Rework on init_codebook_gNB(): More rework (move to MAC...) in !2199 (merged).

  8d338858 - misc: Use inline and c16_t for init_codebook()

4 layers, unitary precoding

sudo ./nr_dlsim -n200 -s35.8 -S40 -x4 -y4 -z4 -e18 -q1 -R106 -b106 -P

	Layer Mapping time	Resource Mapping time
2023.w40	75.43 us	116.79 us
This MR	47.24 us	93.48 us

2 layers, pmi=1

Diff to enable precoding

diff --git a/openair1/PHY/NR_TRANSPORT/nr_dlsch.c b/openair1/PHY/NR_TRANSPORT/nr_dlsch.c
index b04b13ca58..ade0cc8185 100644
--- a/openair1/PHY/NR_TRANSPORT/nr_dlsch.c
+++ b/openair1/PHY/NR_TRANSPORT/nr_dlsch.c
@@ -473,7 +473,8 @@ void nr_generate_pdsch(processingData_L1tx_t *msgTx, int frame, int slot)

           //get pmi info
           uint8_t pmi;
-          if (0 /*rel15->precodingAndBeamforming.prg_size > 0*/)
+          // if (0 /*rel15->precodingAndBeamforming.prg_size > 0*/)
+          if (rel15->precodingAndBeamforming.prg_size > 0)
             pmi = rel15->precodingAndBeamforming.prgs_list[(int)rb/rel15->precodingAndBeamforming.prg_size].pm_idx;
           else
             pmi = 0;//no precoding

sudo ./nr_dlsim -n200 -s35.8 -S40 -x2 -y4 -z4 -e18 -q1 -R106 -b106 -P -p1

	Layer Mapping time	Resource Mapping time	Layer Precoding time
2023.w40	61.01 us	61.77 us	848.95 us
This MR	30.40 us	47.40 us	605.74 us