2.0.0

Add support for GSF v3.09 (#125)

* Add support for GSF v3.09

* Also introduces a pattern for supporting future new versions of GSF.
* Default supported version in the top level gsfpy namespace remains as
  v3.08 for backwards compatibility.
* Provides option for changing default version in top level namespace via
  a DEFAULT_GSF_VERSION environment variable.
* Provides version-specific gsfpy3_08 and gsfpy3_09 namespaces as alternative
  to defining the default GSF version at the top level.

* Include embedded shared object libraries

* Fix typo in CONTRIBUTING.md

* Additional test for GSF version set via environment

* Add test coverage reporting

* Note that the top-level gsfpy namespace is not included in coverage reporting
  as it simply acts as a switch between gsfpy3_08 and gsfpy3_09.

* Additional tests

* Update code examples in README

* Set version to 2.0.0

* Reflect module structure of subpackages in top level namespace

* For improved backward compatibility it must be possible to do
  imports of the form 'from gsfpy.enums import ...'

* Mirror all version specific submodules using generic mechanism

* In order that imports of the form 'from gsfpy.gsfRecords import ...'
  can be used with the version-specific implementations of the package
  it is necessary to mirror those implementations from the top level
  gsfpy namespace. This is now done in a generic way using dynamic
  import, so that future versions of GSF will also be supported without
  any code changes in the top-level gsfpy package being necessary.

* Remove unneeded global

* Fix paths in README

* Fix comment in README

* Rename parameter for improved clarity

* Tidy up test data and associated documentation

* Add comment to Makefile to explain multiple pytest runs