Add eps could lead the same floating point number,
instead resize to the next larger floating point number that is available

... and remove the former.

Based on measurements using identifySubdomains applied to
Danube model with 16x10^6 nodes the new default value allows
for fastes search.

Enforce the sequence of operations by (correct set) parenthesis.

(1) compute difference
(2) compute nextafter
(3) compute quotient
(4) floor

The spdlog is public in BaseLib and repeating it in
on BaseLib dependent libraries is not necessary and
violates the ODR.

The 'next' value is not used afterwards.

The function is identical to the const version and
not modifying.

Removing the return value.