Change comparison syntax

[mdekauwe]

Some of the code uses a functional to compare floats and it some of the more recent quick edits this is no longer true. I should fix this.

Just noticed that as part of C99 we can just use inbuilt macros, so let's do that:

https://www.gnu.org/software/libc/manual/html_node/FP-Comparison-Functions.html

isgreater isgreaterequal isless islessequal

Quick find & replace job.