Import order causes warnings when working with epicscorelibs and softioc

As [epicscorelibs](https://github.com/mdavidsaver/epicscorelibs) maintains its own libca, it provides the option to set the 'CATOOLS_LIBCA_PATH' before using cothread. This can be found [here](https://github.com/mdavidsaver/epicscorelibs/blob/9974875f9cbddd68a3d8dcc358eb4dd6c38b95db/src/python/epicscorelibs/path/cothread.py#L22). This mechanism does not appear to be ideal.

For example, [softioc](https://github.com/dls-controls/pythonSoftIOC) uses this mechanism [here](https://github.com/dls-controls/pythonSoftIOC/blob/dfc3971d91873f7b44c0daadf675022f5da3aa9d/softioc/__init__.py#L11). The problem comes when another program attempts to use both softioc and cothread itself. A simple program such as the following:

```
#! /usr/bin/env python3

import cothread.catools
import softioc
```

will result in the following error (and potentially problematic behaviour):

```
$ ./test_imports.py
/path/to/pipenv/environment/lib/python3.7/site-packages/epicscorelibs/path/cothread.py:18: UserWarning: epicscorelibs.path.cothread must be imported before cothread.catools to have effect
warnings.warn("epicscorelibs.path.cothread must be imported before cothread.catools to have effect")
```

There are two problems:

1. It is not immediately clear what is causing the error
2. If using standard tools such as isort, you need to manually force a particular order of imports to avoid this error. You need to `isort:skip` multiple lines of code

I am not sure why cothread does not use epicscorelibs by default. Would it be possible to implement this?

Alternatively, some kind of conditional import of epicscorelibs could be useful.