Make accessor namespace opt-in

### Description

It is recommended to provide a single accessor under the package namespace. (See https://github.com/pydata/xarray/issues/1080)

Currently, we have multiple callable accessors in the top level namespace of `xarray.DataArray`, but this is not good practice!

### Possible solutions

The best way is to move all these accessors to a single accessor namespace:

| Current | New |
| ----------------- | -------------------- |
| `da.qplot` | `da.er.qplot` |
| `da.qshow` | `da.er.qshow` |
| `da.qsel` | `da.er.qsel` |
| `da.kspace` | `da.er.kspace` |
| `da.modelfit` | `da.er.modelfit` |
| `da.parallel_fit` | `da.er.parallel_fit` |
| `ds.modelfit` | `ds.er.modelfit` |

However, we lose the convenience of calling these functions directly. As a workaround, we should add an opt-in environment variable to enable the top-level accessors. This way, users can choose to use the old syntax if they prefer!