Add calc.write for DFTD3

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The behaviour of the DFTD3 calculator is different from that of other calculators since it takes a calculator as input. Here, we add DFTD3.write() method to the calculator class to write its child calculator files. First, the child calculator is checked for a write method. If it does, calc.write then returns calc.dft.write by passing args and kwargs. If not, an AttributeError is returned. This gives consistency among calculators when running workflows.

Checklist

• I am familiar with ASE's contribution guidelines.
• Doc strings in code changed in this MR are up to date.
• Unit tests have been added for new or changed code.
• Issue is resolved via "closes #XXXX" if applicable.