Inconsistency in passing attribute for set_mattr and set_sattr in zero_engine branch

[KADichev]

In https://github.com/Algebraic-Programming/LPF/blob/zero_engine_MR/include/lpf/tags.h, there are inconsistencies:

• https://github.com/Algebraic-Programming/LPF/blob/zero_engine_MR/include/lpf/tags.h#L459 has attribute passed by value
• https://github.com/Algebraic-Programming/LPF/blob/zero_engine_MR/include/lpf/tags.h#L396 has attribute passed by reference

[anyzelman]

Closed in commit 1985894