Piecewise version of loadExternalData

[nselliott]

Summary

• This PR is a feature
• It does the following (modify list as needed):
  • Adds overloaded version of IOManager::loadExternalData, providing a way to load external data for only a part of the Sidre hierarchy.
  • Addresses Allow IOManager to load external data at specific locations in Sidre hierarchy. #1479

The pre-existing usage pattern for external data is a three step process:

1. Call IOManager::read() to execute parallel IO to read data from a file and load it into a Sidre Group.
2. Directly set the addresses for every View in the hierarchy that has external data. The calling code is responsible to ensure that the addresses point to validly allocated data of the correct size.
3. Call IOManager::loadExternalData() with the same root file and the same Group that was loaded in Step 1.

This added usage pattern changes steps 2 and 3. For step 2 the calling code sets the addresses for a subset of the Views with external data., then for step 3 it calls the new overloaded IOManager::loadExternalData on a Group somewhere in the hierarchy that contains the Views touched in step 3. Steps 2 and 3 can be repeated to set addresses and load data into external Views at different locations in the hierarchy.

[rhornung67]

why is parent group needed here?

[nselliott]

In order to find the needed data in the HDF file, the method needs the relative path from parent_group to load_group. The logic to figure this out is inside the method. The alternative would be to force the user to construct this relative path and pass it in.

[rhornung67]

Got it. Thank you for the explanation.

[rhornung67]

Some comments about the comments....