Allow for mounted tool rpy origin to be defined in mounting plate xacro

[niko-holmes]

Proposed changes

RPY of mounted tool can now be specified when calling mounting plate xacro. This is needed as part of shadow-robot/dx_system#44

Types of changes

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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)

Checklist

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• Read and follow the contributing guidelines.
• Checked that all tests pass with my changes
• Added tests (automatic or manual) that prove the fix is effective or that the feature works
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• Tested on real hardware (if appropriate)