Added static casts of size_t container sizes

[w1th0utnam3]

Previously size_t values returned by the internal containers were implicitly converted to the ECS size_type which results in compiler warnings about possible loss of accuracy (if size_type < size_t). If you want to make the user aware of this issue, I would rather add an assert in the create() method or something like this.

[skypjack]

At the end of the day, the type of the entities rules on the size and the size type. I added Entity template parameter and forgot to clean up that part. I don't get this PR solves the issue, it's simply a workaround. Let me review it entirely and propose something different.

[w1th0utnam3]

I was a little bit confused why size_type is currently fixed to std::uint32_t. And yes, obviously just a workaround - probably not sensible for this general repository in comparison to the toy project I use it for.

[skypjack]

I pushed on master a few changes that should clean things up. Can you give me a feedback, please?

[w1th0utnam3]

1. I get the internal compiler error of MSVC again in the base class initialization of ComponentPool (here). I have to experiment to find a workaround.
2. There is still a potentially lossy conversion here (std::size_t to pos_type)

[skypjack]

Oh, I'm sorry, appveyor is having problems for I didn't set master as default branch. I'm going to fix it, not a problem. Give me a couple of minutes.

[w1th0utnam3]

The problem is the usage of the alias template

template<typename Comp>
using Pool = ComponentPool<Entity, Comp>;

in the pack expansion context of the initializer:

explicit ComponentPool(size_type dim = 4098) noexcept
        : Pool<Component>{dim}, Pool<Components>{dim}...

Replacing it with the full class name:

explicit ComponentPool(size_type dim = 4098) noexcept
        : ComponentPool<Entity, Component>{dim}, ComponentPool<Entity, Components>{dim}...

fixes the error.

[skypjack]

Push upstream and added -Wconversion to the CMakeLists.txt. From now on, travis will tell us what's wrong in terms of conversions. ;-)

[w1th0utnam3]

Cool. Yes, everything seems fine now.

[skypjack]

Thank you very much. I'm closing this issue and considering it solved.