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This closes issue #23

In case of single precision calculations the stripe_width needs to
be a multiple, which differs from the double precision by a factor of 2
since one needs 32 bytes alignment and the sizeof(float) and sizeof(double)
is different by a factor of two

This commit closes issue #18

library

It the configure option "--enable-single-precision" is specified,
ELPA will also be build for single precision usage. The double precision
and single precision will be available at the same time with names
"solve_evp_real_1stage_double" or "solve_evp_real_1stage_single" and
so on...

This change immplied some major refactoring of the ELPA code:
1.) functions/procedures had to be renamed with suffix "_double"

2.) If necessary the same functions have to be available with suffix
"_single"

3.) Variable kind definitions have to be consistent with the
intented use

To avoid uneccessary code duplication this is done (most of the time)
with preprocessor string substitution.

The documentation has been updated.

NOT SUPPORTED are at the moment:

- single precision usage of ELPA2 with kernels, others than "generic"
  and "generic_simple"

- single precision usage of GPU