and passing NULL from the api

To be used like this

   class(elpa_t), pointer      :: e
   class(elpa_autotune_t), pointer :: tune_state

   e => elpa_allocate()
   call e%set(...)
   [...]
   assert_elpa_ok(e%setup())

   tune_state => e%autotune_setup(ELPA_AUTOTUNE_FAST, ELPA_AUTOTUNE_DOMAIN_REAL)

   ! Autotuning loop, continues until all combinations have been tried
   do while (e%autotune_step(tune_state))
     ! Do the steps that are representative of your calculation
     call e%eigenvectors(a, ev, z, error)
   end do

   ! Fix best parameters, and de-allocate the autotune object
   call e%autotune_set_best(tune_state)
   call elpa_autotune_deallocate(tune_state)

The PGI compiler (of course) complained about a missing module
(elpa_generated_fortran_interfaces.mod) when compiling the test
programs.

It is true (in a way) that some part of this module is indeed necessary,
as the public-facing function signatures have arguments that are
referring to those three functions in their type:

  elpa_strerr_c(elpa_error)
  elpa_int_value_to_string_c(name, value, string)
  elpa_int_value_to_strlen_c(name, value)

Thus, for these three we create another header prefix,

  !pf>

for Fortran definitions that should be public. Those are included in
elpa_api.F90.

These can be used internally by

  call self%set_private(name, value)

   or

  call self%get_private(name, value)

but are not valid for the self%set()/get() routines, i.e. are not
visible to users of ELPA.

We decided not to provide this information via the key/value store, but
rather to use the ftimings module for that.

@loh will take care to implement ftimings in the new API
@amarek wants to port the legacy API so that the old global module
  variables still contain valid values

The legacy API is has been (internally) ported to use the new interface.
The intent is that users of the legacy API do not have to change their
codes.

Next step is to completely adapt the .gitlab-ci.yml file

Still missing is the compatibility layer, currently it only compiles
when configure is called with

  --disable-legacy

Also, a more general solution to parameter passing via environment
variables would be nice.

Supports multiple indexes now, to be used for non-tunable 'parameters'