Implement Astropy Quantities

[aphearin]

Mostly this will only impact the halo catalog and merger tree dtype formatting.

[aphearin]

Give users the option to write supported model components that either do or do not work with Quantities. Try to implement this via a class-level use_quantities attribute. If set to False, the composite-model decorator will ensure that an ndarray view of the columns are passed to the component, and then the component model can safely perform arbitrary operations on the columns; if set to True, the passed columns will be Quantities, and so the component model will need to make columns dimensionless if they want to perform nonlinear transformations on them. Suggestion due to @eteq.