not all 'region of integument' children should be 'multicellular structure'

[Clare72]

Currently 'organ system' and 'organ system subdivision' are both under 'multicellular structure'
'integumentary system' and 'region of integument' are therefore both 'muliticellular structure'

For 'integumentary system', I think the cells that secrete the cuticle are included, so this is probably fine and I think all 'organ system's will include some cells, so I would not change anything here.
However, 'region of integument' has acellular child terms, e.g. 'sclerite', which should not be 'multicellular structure' and it is possible that not all subdivisions of every 'organ system' will contain multiple cells.

Options:

1. Move 'organ system subdivision' out from under 'multicellular structure' to 'anatomical structure'
   'organ system subdivision' classes that belong under 'multicellular structure' will need to have this as an additional parent

2. Review whether 'region of integument' terms are correctly classified here.
   Definition of 'region of integument':

A region of cuticle and its underlying epidermis.

I think many of the child terms don't include epidermis. The ones that do (and include more than one cell) are fine as 'multicellular structure'. Others should perhaps be 'cuticular specialization' (which is 'acellular anatomical structure')

There may be a problem here if we need a structure that is secreted by a single cell and we want to include the cell as part of the structure. Not sure if there are currently any examples of this.

3. Both of the above

Currently leaning towards doing (2) for now, but bearing in mind that we may need to do (1) at some point if we need to define any subdivisions with <2 cells.
In any case 'organ system subdivision' should probably also be SubClassOf 'part of' some 'organ system'

related to #1124