moved reactor to start from floor level

[shimwell]

Proposed changes

Changes the reactor to start at floor level, this makes comparison simulations with CSG easier and location of the neutron source easier

Types of changes

What types of changes does your code introduce to the Paramak?
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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code refactoring
• Documentation Update (if none of the other choices apply)
• New tests

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)