fix(idm): stress package loaders allocate duplicate iper/ionper variables

[mjreno]

Issue introduced in PR #1610.

• This PR moved iper/ionper to dynamic loader base class
• There are 2 contexts (loaders) for stress data- an input style specific block loader (e.g. list, array) and the generic dynamic loader that manages the load and allocates the input style loader. These were both allocating iper/ionper via the base class.
• Fix is to move iper/ionper back into the higher level dynamic loader.

[mjr-deltares]

Hi @Manangka this is related to the problem you find in your work with #1701