Move wrapper load_qasm_* methods to a submodule

[delapuente]

Motivation and Context

It was intended that load_qasm_string and load_qasm_file call static constructors in the QuantumCircuit instead of being implemented at wrapper module-level.

How Has This Been Tested?

Passing the same test suite without alterations.

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[delapuente]

Per a discussion with @atilag we agreed on moving these methods again to wrapper but to a different submodule so QuantumCircuit class remains as lean as possible. Utilities and higher abstractions should live outside the core.

[diego-plan9]

For the new files, can you adjust the license header so it follows the format recently introduced by #550 ?

# -*- coding: utf-8 -*-

# Copyright 2018, IBM.
#
# This source code is licensed under the Apache License, Version 2.0 found in
# the LICENSE.txt file in the root directory of this source tree.

# pylint: disable=something-only-if-there-is-no-other-way

"""
Description of the module ...
"""
from qiskit import ...

[CLAassistant]

All committers have signed the CLA.

[atilag]

@delapuente can you update CHANGELOG.rst accordingly? :)

[diego-plan9]

Thanks, @delapuente !