add complex support

[henry-ald]

While the existing CosmicStringEmulator object has a complex option cplx, this is only applied to the PyWPH components of the code. The Scipy optimization does not support complex inputs/outputs.

• Adds wrapper for converting between complex and flattened real views of an array
• Adds wrapper for converting between flattened real views and flattened complex views of an array
• Adds necessary flow control dependent on cplx=True