MAINT: use random Generator instead of RandomState

[mdhaber]

It looks like the intent was to switch from use of RandomStates to Generators, but the transition is incomplete. I searched for occurences of random.rand (which also finds randn and randint) and replaced them.

@tupui would you check me here? Some things to look for:

• rng is defined where it's used
• rng is defined only once (i.e. I didn't add it a new one where it was unneeded)
• random.rand with separate positional arguments has been replaced with rng.random with single positional argument
• random.randn with a positional arguments has been changed to rng.normal with size keyword argument, etc.
• random.randint has been replaced with rng.integers (signatures are compatible, right?)
• I didn't miss anything obvious?

[tupui]

I did not see any obvious miss no 😃