Add tests for new FieldSet object

[VeckoTheGecko]

• Chose the correct base branch (v4-dev for v4 changes)
• Fixes none
• Added tests

Changes:

• testing:
  • Add U and V fields to example datasets
  • Add fieldset tests (and stubs for future tests)
• cleanup
  • Remove reference to _parent_mesh
  • Split reprs into a separate file
• code
  • Add check for field type in FieldSet.add_field
  • Add fieldset.constants (avoids call to setattr)
  • Add FieldSet.__getattr__
  • Fix FieldSet.add_constant_field

[VeckoTheGecko]

Any more tests we should have in here @fluidnumerics-joe ? (even if its just stubs def test_my_thing():... so that we don't forget them later)

[fluidnumericsJoe]

Taking a look right after the elphe meeting this morning

[fluidnumericsJoe]

Just a couple minor changes

[VeckoTheGecko]

Just a couple minor changes

I've responded now :)

Any more tests we should have in here @fluidnumerics-joe ? (even if its just stubs def test_my_thing():... so that we don't forget them later)

Let me know if anything comes to mind.

Good to merge?