Add LV::Palette methods for iterations and comparisons

[kaixiong]

This PR implements:

• standard C++ container methods such as begin() and end() for iteration
• equality and inequality operators for comparing palettes

[hartwork]

I'm likely missing something, but I do not see anything in the first commit that would need <algorithm> and also not in the second. What is <algorithm> used for/by in the pull request?

[kaixiong]

You're right, it's not used. I think I missed this when splitting the commit out.

[hartwork]

While these two seem to help readability further down in some regard, the fact that the color aspect — a key aspect of the return type — is hidden away seems (worse than hiding the vector away) and like a disservice to me. Maybe const_color_iterator? Is there a concept other than D.R.Y. and brevity here that I am missing?

[kaixiong]

Type erasure is kind of the point for generics. These type definitions are part of LV::Palette's interface for generic programming (think protocols in Python duck typing) and are standard in C++ containers.

For example, imagine a generic function that works on some sequence of LV::Color objects. Say, computing the average colour intensity of the collection. The algorithm doesn't know or want to know if it's a std::vector<Color>, a std::array<Color>, a C array (LV::Color *), LV::Palette or some fancy structure that does size optimization tricks. All it cares about is iterating, calculate each color's intensity, accumulating and finally averaging in the end.

So in that function, when it needs to declare and instantiate an iterator for the container, it can declare Container::iterator, Container::const_iterator as its type, where Container could be any of the above.

[hartwork]

@kaixiong by Container you mean Palette I assume? I think I understand where you are going with this. I will approve the pull request and only removal of <algorithm> remains to be changed…

[kaixiong]

@hartwork, hmm upon some more thinking, perhaps I should also provide typedefs for value_type (= Color) and maybe reference (Color &) and const_reference (Color const&). This also serves as documentation what the element type is when iterating over an LV::Palette instance. I don't really want to implement the full SequenceContainer interface just yet though.