This patch adds ability to get context schema node from which when
contition is evaluated. It also fixes memory leak of original
when_conditions

Signed-off-by: Stefan Gula <steweg@gmail.com>

This patch adds missing Enum class to __init__.py to allow imports by
library users

Signed-off-by: Stefan Gula <steweg@gmail.com>

This crash (abort) is hit when the parsed data does not validate:

    python3: ..validation.c:1998: lyd_validate: Assertion `tree && ctx' failed.
    Aborted

The `tree` mentioned in the assert is from last value ffi.NULL that was passed
in in the `parent is not None` case.

Instead just pass a pointer for a return value regardless of the parent value.

Signed-off-by: Christian Hopps <chopps@labn.net>

includes the default context options LY_CTX_ALL_IMPLEMENTED and
LY_CTX_ENABLE_IMP_FEATURES into the ctor of Context class.

Signed-off-by: Esben Laursen <hyber@hyber.dk>

The arg is added as `str` but the fmt requires `int`, causes exception when
turning on python logging -- fix.

Signed-off-by: Christian Hopps <chopps@labn.net>

When one wishes to disable logging temporarily (e.g., when calling API functions
when an error result is expected and OK), libyang provides
ly_temp_log_options(). We need access to this in python, so export access to the
function and add a python context (i.e., "with") manager API for using
it idiomatically as well.

ex usage:

```
    with temp_log_options(0):
        ly_unwanted_logging_call();
```

Signed-off-by: Christian Hopps <chopps@labn.net>

Add a new log unit test. Add a test for the newly exposed
temp_log_options() function as well as the already implemented
configure_logging() function.

Signed-off-by: Christian Hopps <chopps@labn.net>