lhef: reduce number of allocs in Decoder

[dolmen]

In the lhef.Decoder, allocate slices using sliced bigger chunks in order to reduce the number of allocations.

Example:

a := make([]int32, n)
b := make([]int32, n)
c := make([]int32, n)

is replaced with:

n32 := make([]int32, 3*n)
a := n32[:n:n]
b := n32[n : 2*n : 2*n]
b := n32[2*n:]

[sbinet]

LGTM

thanks for improving this. (it feels like good old FORTRAN column blocks, but hey :P)