Updated to revision 3.7.4.1 for release.
Merge branch 'maint'

Used merge strategy -s ours to skip release commit.
Updated revision to 3.7.5 for release.
Updated revision post-release.
volk: adding tanh kernel with support for sse and avx.

There are two generic kernels, one using tanh and another using a series approximation. The SIMD code uses the series approximation.