rename reductors

[sdrave]

@JuliaBru told me that pymor.reductors.linear.reduce_stationary_affine_linear and
pymor.reductors.stationary.reduce_stationary_coercive are not very intuitive names, and I agree. How should we name them (@ftalbrecht)?

[sdrave]

Maybe reduce_coercive and reduce_coercive_simple in pymor.reductors.coercive?

[ftalbrecht]

What you propose could do. I do not have a better name at the moment...

[sdrave]

Done in 16a56cb.