Default halo catalogs should have num_ptcl_per_dim attribute

[aphearin]

The following line of code taken from the docstring of delta_sigma should not raise an exception:

total_num_ptcl_in_snapshot = halocat.num_ptcl_per_dim**3

Instead this raises:

AttributeError: 'CachedHaloCatalog' object has no attribute 'num_ptcl_per_dim'

[aphearin]

Likewise, halocat.cosmology also triggers an error