ndarray extension to fields

Nobody has claimed this yet.

a few ideas I don't want to lose, so throwing them up here:

using a ND array to define a field on a 2D structured mesh where assoc between field values and the elements is fully specified (vs blueprints implicit i,j relationship)

These examples are for a 10x10 element mesh:

scalar field:

```
field:
values: [ lots of values]
topology: "topo"
association: "element"
shape: [ 10, 10]
indices: [ "i","j"] # order corresponds to shape above, names matter
strides: [ blah,blah] # optional
```

vector field:

```
field:
values: [ lots of values]
topology: "topo"
association: "element"
shape: [ 10, 10, 3 ]
indices: [ "i", "j", "v"] # order corresponds to shape above, "i" and "j" names matter
strides: [ blah,blah,blah] # optional
```

using a ND array to define a field o on an unstructured mesh where assoc between field values and the elements is fully specified (vs implicit id ->element relationship)

These examples are for a 100 element mesh:

scalar field:

```
field:
values: [ lots of values]
topology: "topo"
association: "element"
shape: [ 100]
indices: [ "id" ] # "id" name matters
strides: [ blah,blah] # optional
```

vector field:

```
field:
values: [ lots of values]
topology: "topo"
association: "element"
shape: [ 100, 3 ]
indices: [ "id","v"] # "id" name matters
strides: [ blah,blah] # optional
```

The issue names no source files, tests, or entry points. Start by reviewing the existing field and topology representation against the scalar and vector YAML examples; done requires a decided, fully specified ndarray model for structured and unstructured meshes.

Written by the indexing model from the issue text.