Fix: Throw a FieldError exception when get_value() fails

Throw an exception whenever bt_ctf_field_get_error() reports an error.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Add support for floating point fields in the python bindings

get_value() now returns a floating point field's value.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Add support for variant fields in the python bindings

get_value() now returns the variant's selected field's value
when called on a variant.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Add support for structure fields in the Python bindings

get_value() returns a dictionary when the field is a structure.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>