Update default global system solver parameters for paralell-in-time

Issue/feature addressed

In Nektar++, a direct solver is used by default for serial cases while a iterative solver is used when multiple processors are used. However, the current implementation does not take into consideration time-parallelism. This MR is partially split from !1613 (merged).

Proposed solution

Update code to take into consideration time-parallelism by using an iterative solver as the default option only for cases with space-parallelism.

Implementation

Tests

Suggested reviewers

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Notes

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Checklist

• [ ] Functions and classes, or changes to them, are documented.
• [ ] User guide/documentation is updated.
• Changelog is updated.
• [ ] Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• [ ] License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).