Fix cropping when omitting a limit

[gabrieldansereau]

Cropping a layer while omitting to specify one of the limits did not work properly. For example:

julia> temp = worldclim(1);
julia> temp[left = -145.0, right = -50.0, top = 75.0, bottom = 20.0];
julia> temp[left = -145.0, right = -50.0, top = 75.0];
ERROR: MethodError: no method matching #getindex#2(::Float64, ::Float64, ::Float64, ::Nothing, ::typeof(getindex), ::SimpleSDMPredictor{Float64})
Closest candidates are:
  #getindex#2(::K, ::K, ::K, ::K, ::typeof(getindex), ::T) where {T<:SimpleSDMLayer, K<:Union{Nothing, AbstractFloat}} at /home/gdansereau/github/SimpleSDMLayers.jl/src/lib/overloads.jl:166
Stacktrace:
 [1] top-level scope at none:0

I fixed it with two tweaks to the getindex overload (the match_longitude calls and the types).

While at it, I also added an overload to crop based on a NamedTuple. I think it's useful as it allows to crop with a variable, which can be reused easily.

coords = (left = 145.0, right = -50.0, top = 75.0, bottom = 20.0)
temp[coords]

I've added tests for both of these.

[gabrieldansereau]

Is there a better way to make the types work? They have to be "independent", so that one can be Nothing while the others are AbstractFloat. The previous code only worked if they were all Nothing or all AbstractFloat.

[tpoisot]

Well, we can always not be specific about the types here, since there is an isnothing on one side, and the getindex measures do not have methods for anything other than a float in this case... would it fix the tests?

[gabrieldansereau]

Oh, it already works like this. I was just wondering if there was a cleaner way to call the types.
It works the same if we're not specific about them, as match_longitude is specific for floats.

[tpoisot]

I think this should simplify the declaration substantially

[gabrieldansereau]

Yes, that's great!