MNT: change bilby MCMC to use glasflow instead of nflows

Closes #729 (closed)

I also took this chance to switch the tests to use pytest-requires which will mark tests that require glasflow as skipped rather than passing when it is not installed.

This will also require a change to the images, so we should be slighlty careful when merging this.

bilby/bilby_mcmc/flows.py

 import torch
-from nflows.distributions.normal import StandardNormal
-from nflows.flows.base import Flow
-from nflows.nn import nets as nets
-from nflows.transforms import (
+from glasflow.nflows.distributions.normal import StandardNormal
+from glasflow.nflows.flows.base import Flow
+from glasflow.nflows.nn import nets as nets
+from glasflow.nflows.transforms import (
     CompositeTransform,
     MaskedAffineAutoregressiveTransform,
     RandomPermutation,
 )
-from nflows.transforms.coupling import (
+from glasflow.nflows.transforms.coupling import (
     AdditiveCouplingTransform,
     AffineCouplingTransform,
 )
-from nflows.transforms.normalization import BatchNorm
+from glasflow.nflows.transforms.normalization import BatchNorm
 from torch.nn import functional as F
 
 # Turn off parallelism