use `Base.@kwdef` for setting up defaults for constant-holding structures

[exaexa]

instead of

mutable struct someconfig
  x :: Int
end

default_someconfig = someconfig(1234)

do

Base.@kwdef mutable struct someconfig
  x :: Int = 1234
end

...this also makes seriously nice constructors, so that users can do:

someconfig(some_particular_nondefault_variable=4312)

[exaexa]

@stelmo this is now done in FROGs, is there any plan to migrate the memote stuff?

[stelmo]

Not yet, but there should be! Let's keep this issue open until I have done that