Fix/type stability

[olivierbonte]

Suggestions to improve type stability of some functions. .

[bgctw]

Thanks Oliver for those suggestions. Can you, please, change some of the hard constructors to the suggested usage of convert?, And can you, please, resolve the conflict in unit_conversions.jl, before I merge it to main?

[olivierbonte]

Thanks for the feedback, conflict is resolved and as as suggested changed use of FT(...) to convert(FT,...).

[bgctw]

For constants I prefer the FT(...) over the convert, then the Julia compiler inserts the correctly typed value directly in the compiled code, e.g. FT(0.3). If you you are not bored by this picky issue, please, change those back to the more readable hard cast. Otherwise, I can merge it the way it is.

[olivierbonte]

Sorry for late reply, was planning on changing back to FT(...) now, but I see the PR is already merged. Still made the commit olivierbonte@eabb4be. Made a new PR just in case you still want to adapt it to be this way, otherwise feel free to decline the new PR, found at #58