RationalQuadraticSpline for batch dimension

[mmikhasenko]

This PR enables using RationalQuadraticSpline with Couplings on matrix dimension

b = RationalQuadraticSpline(rand(11,5), rand(11,5), rand(11,5)) # 11 nodes, batch of 5
b(rand(1, 5)) # should return matrix of dim (1, 5) to be able to `combine`