Make healpy and dustmaps optional dependencies

[kelle]

healpy and dustmaps can be challenging to install and don't provide a core functionality (#118). Maybe we should consider making them an optional dependency. I'm not sure what code would need to be modified in order to implement this.

https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#dependencies-optional-dependencies

[hover2pi]

Removing this as default functionality introduces this warning when initializing an SED: