mat_str_to_expr: refinement for ,, some error

[MatthieuStigler]

Check here:

library(matPkg)
plot(1)
title(mat_str_to_expr("N=4 change strength of spike (p^alpha)"))

title(mat_str_to_expr("N=4, change strength of spike (p^alpha)"))
#> Error in parse(text = str_prep): <text>:1:4: unexpected ','
#> 1: N=4,
#>        ^

^{Created on 2020-10-02 by the reprex package (v0.3.0)}