Replace depricated calculate_com function

[semaphoreP]

it looks like the latest version of REBOUND no longer has the calculate_com() function and this causes our tests to fail. Replacing with the com() function.

[coveralls]

Pull Request Test Coverage Report for Build 7452945327

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage remained the same at 86.488%

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Files with Coverage Reduction	New Missed Lines	%
orbitize/read_input.py	2	68.78%

Totals
Change from base Build 6242083472:	0.0%
Covered Lines:	5204
Relevant Lines:	6017

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💛 - Coveralls