replace checks that see if two float variables are equal via `==` with something resiliant to floating point errors/roundoff

[ChrisZYJ]

== is used to compare floating point numbers in a few places, like:

elseif (polydisperse .and. (poly_sigma == dflt_real)) then

SHB addendum: Equality checks of floats against floats is bad practice and fragile. Replace with something more robust.

[sbryngelson]

related to #488