Simplify code with `merge` in stdlib_stats_corr.f90, stdlib_stats_cov.f90 and other files

In https://github.com/fortran-lang/stdlib/blob/stdlib-fpm/src/stdlib_stats_corr.f90

I see many lines using `merge` such as

```fortran
mask_ = merge(.true., .false., mask(:, i) .and. mask(:, j))
```

I think this can be written as just

```fortran
mask_ = mask(:, i) .and. mask(:, j)
```