DoDemul segfault with 44 antennas and AVX512

[asedlmayr]

Suspected to be alignment issue on DoDemul line 141 ( mm512_store_ps(dst + j * cfg->BsAntNum() * 2, data_rx);)

If antenna_num=48 (46/42) everything appears to work ok ....
-change antenna_num to 44 and you should see a segfault.

[di-mu]

Change the condition "cfg_->BsAntNum() % 4 == 0" to "cfg_->BsAntNum() % 8 == 0" in https://github.com/jianding17/Agora/blob/d090d16d7e7ae1fc91df6caf3a4996769a9c0e97/src/agora/dodemul.cc#L118 can fix this segfault if antenna_num is 44.

It seems mm512_store_ps() only allows the destination offsets to be multiplications of 16.
https://github.com/jianding17/Agora/blob/d090d16d7e7ae1fc91df6caf3a4996769a9c0e97/src/agora/dodemul.cc#L141

[asedlmayr]

In member function ‘virtual EventData DoDemul::Launch(size_t)’:
/scratch/repos/agora/src/agora/dodemul.cc:140:65: warning: pointer of type ‘void *’ used in arithmetic [-Wpointer-arith]
140 | : _mm512_i32gather_ps(index, src + j * 2, 4);

don't add to void pointers. cast the memory before doing a jump

[asedlmayr]

I made an update in #338 to address the issue. Hope we can understand it better and fix it in a more rigorous way.