Use type member insted of type parameter for Entity and EntityId mapping

[gakuzzzz]

Hi all,

It is unnecessary to use type parameter for Entity and EntityId mapping.
Use of this type argument destroys the encapsulation, so the use of type members is recommended.

Diff indicates that you can write briefly on the user side.

Best regards.

[sp1rytus]

Thanks for your suggestion.
Certainly it is possible to reduce extra declaration by constraining the type of Id with EntityModel.
However, the amount of modification of the existing code also increases,
so we will consider it as a merge to the next ixias v.1.2.x system.
I will merge later.