v0.2.1

Lattice_qcd_rs

• add const on multiple functions where applicable
• update nalgebra to 0.31.0
• update num-traits to 0.2.15
• update rayon to 1.5.3

Lattice_qcd_rs-procedural_macro

• update quote to 1.0.19
• update syn to 1.0.98
• update proc-macro2 to 1.0.39