Merge branch develop into non-orthogonal-maps

For those of you following, the merge of develop into the
non-orthogonal-maps might come as a surprise. At the OST
developer workshop we have agreed that we would cherry-pick
some of the functionality of the non-orthogonal-maps branch
into develop. So why the merge now? The reason for the merge
is simple: One of my projects depends on the non-orthogonal-
maps branch and some features that are only available in
current develop.

Conflicts:
	modules/base/pymod/wrap_base.cc
	modules/base/src/string_ref.cc
	modules/base/src/string_ref.hh
	modules/base/tests/CMakeLists.txt
	modules/base/tests/test_string_ref.cc
	modules/geom/pymod/export_mat2.cc
	modules/geom/pymod/export_mat3.cc
	modules/geom/pymod/export_mat4.cc
	modules/geom/pymod/export_vec2.cc
	modules/geom/pymod/export_vec3.cc
	modules/geom/pymod/export_vec4.cc
	modules/geom/src/vecmat3_op.cc
	modules/geom/src/vecmat3_op.hh
	modules/geom/tests/CMakeLists.txt
	modules/geom/tests/test_vec3.cc
	modules/gfx/src/map_iso.cc
	modules/gfx/src/map_iso.hh
	modules/img/alg/tests/test_fft.cc
	modules/img/alg/tests/test_transform.cc
	modules/img/alg/tests/tests.cc
	modules/img/base/src/point.cc
	modules/img/base/src/size.cc
	modules/img/base/tests/test_image.cc
	modules/index.rst
	modules/io/src/mol/pdb_reader.cc
	modules/io/src/mol/pdb_writer.cc
	modules/io/src/mol/star_parser.cc
	modules/io/tests/test_io_pdb.cc
	modules/mol/base/src/impl/entity_impl.hh