Add support for plane strain triangle elements CPE3

[ngrilli]

Would it be possible to add support for reading CPE3 elements from Abaqus?

I think it should be just about adding a line:

"CPE3": "triangle",

after

"CPS3": "triangle",

in _abaqus.py

It would help a lot in my project where I need to read both types.

At the moment I get a:
Element type not available: CPE3
error message.

Thank you very much,
Best Regards,
Nicolò