Estep_girt_mg: Estep in girt.
In takuizum/irtfun2: Some useful functions for IRT

Estep in girt.

1	Estep_girt_mg(x, a0, b0, Xq, AX, Yr, BY, D, group, ind, resp, MLL)

`x`	DataFrame.
`a0`	discrinimation parameter vector.
`b0`	difficulty parameter vector.
`Xq`	node of theta dist.
`AX`	weight of theta dist.
`Yr`	node of phi dist.
`BY`	weighit of phi dist.
`D`	factor constant.
`group`	a vector.
`ind`	a design matrix for group.
`resp`	a design matrix for person.
`MLL`	a vector