update `as_array` usage

[casperdcl]

Now that #1314 is merged, we should ^discussion:

• change as_array -> asarray as far as possible
• possibly add a default argument as_array(..., warn=True)
  • manually invoke as_array(..., warn=False) to suppress warnings where intended
  • alternatively use warnings.catch_warnings() @contextmanager to temporarily suppress

[paskino]

In CIL we started using as_array because only then we could have access to a numpy array for a number of tasks, such as regularisation. The consequences of the warning message will go beyond SIRF. I suggest we talk about this with CIL devs and agree on a path.

[KrisThielemans]

Are you sure you meant as_array? I wouldn't understand why you go to the "old" version.

[paskino]

It's been years we've been using as_array.

[KrisThielemans]

The Array API says to use asarray(). For SIRF, it will be vastly more efficient. So, at some point, CIL should move to asarray().