Implement "Fessward" modifier

This is probably best treated as a specific arrangement, in which the objects are arranged in annulo but pointing towards the centre. Would need to know which was the "top" of a charge (see other issue about charge orientation)

首先查看与 charge 方向相关的 issue，因为该 issue 对 charge 的顶部尚未作出定义。定义“Fessward”修饰符应如何表示 charges 指向中心的 annulo 布局，包括方向语义；完成的标准是确定一致认可且可测试的行为。

由索引模型根据 Issue 内容生成。