loh/fix elpa index

See merge request !10

lfind() expects a proper comparison function where the order of the
arguments does not matter. Previously it was assumed that the first
argument is always a pointer to an unspecific "key" object and the
second argument a pointer to an array member to compare against.

However, the standard requires the two arguments to be interchangeable
pointers to array-member like objects.

Surfaced as musl-libc does in fact call it the other way around,
compared to glibc.

Fix problem in elpa2_print_kernels with GPU kernel

See merge request !9

The GPU kernel can only be set, if e%set("use_gpu",1) has been set
before and GPU usage was thus requested.

In the elpa2_print_kernels, this was not reflected, and the GPU kernels
were checked via can_set method without activating GPU usage
=> GPU kernels were always reported as non exisitent

by allreducing the measured time and using average over ranks insted

to avoid stalling between storing and loading parameters