Get Omega^-1 and derivatives

## Arguments

- invObjOrMatrix
Object for inverse-type calculations. If this is a matrix, setup the object for inversion

`rxSymInvCholCreate()`

with the default arguments and return a reactive s3 object. Otherwise, use the inversion object to calculate the requested derivative/inverse.- theta
Thetas to be used for calculation. If missing (

`NULL`

), a special s3 class is created and returned to access`Omega^1`

objects as needed and cache them based on the theta that is used.- type
The type of object. Currently the following types are supported:

`cholOmegaInv`

gives the Cholesky decomposition of the Omega Inverse matrix.`omegaInv`

gives the Omega Inverse matrix.`d(omegaInv)`

gives the`d(Omega^-1)`

withe respect to the theta parameter specified in`thetaNumber`

.`d(D)`

gives the`d(diagonal(Omega^-1))`

with respect to the theta parameter specified in the`thetaNumber`

parameter

- thetaNumber
For types

`d(omegaInv)`

and`d(D)`

, the theta number that the derivative is taken against. This must be positive from 1 to the number of thetas defining the Omega matrix.