Read and write Strahlkorper coefficients from disk

[nilsvu]

We need to load Strahlkorper YLM coefficients from a file to initialize the shape of our excision surfaces, both for high-spin inspirals and for ringdowns.

[knelli2]

For writing, we'll need to write the coefficients from ObserveSurfaceData.hpp to an h5 dat file. For reading, currently we'll want the ability to read coefficients in the Sphere and BCO domain creators. We'll probably want support for prolonging/restricting the coefficients to the input l_max when reading.

[nilsvu]

@macedo22 I heard that you are working on this?

[macedo22]

Yes I'm currently trying to learn a bit about how we do IO by poking around some files and plan on meeting with @knelli2 to understand what is wanted in a more concrete sense / how and where this IO needs to happen.

[nilsvu]

Ok great!

[nilsvu]

@macedo22 please let us know if you need assistance, since this is an important task for progress :)