Prefoil dependency

Description

#141 added a hard prefoil dependency but setup.py was not updated to account for this. All tests that import pyGeo fail if you do not also have prefoil installed (which includes some ADflow tests).

At the very least, prefoil should be added in install_requires. However, I don't see much use in making everyone who wants to use pyGeo install prefoil. I would prefer if DVGeometryCST was imported like the other classes that have non-standard dependencies:

pygeo/pygeo/__init__.py

Lines 11 to 22 in d786ca8

    
           try: 
        
               from .parameterization import DVGeometryVSP 
        
           except ImportError: 
        
               pass 
        
           try: 
        
               from .parameterization import DVGeometryESP 
        
           except ImportError: 
        
               pass 
        
           try: 
        
               from .parameterization import DVGeometryMulti 
        
           except ImportError: 
        
               pass

Similarly, if DVGeometryCST cannot be imported, we should skip its tests.

We can use this issue to discuss what the best approach is.

I'm happy to do the latter where DVGeometryCST is imported using the try/except block. Shall I make a PR with the change and we can discuss from there?

Sounds good. Thanks!

Ok, working on it now. Sorry about the oversight!

	try:
	from .parameterization import DVGeometryVSP
	except ImportError:
	pass
	try:
	from .parameterization import DVGeometryESP
	except ImportError:
	pass
	try:
	from .parameterization import DVGeometryMulti
	except ImportError:
	pass