Attributes/datasets with dtype: "text" and enumeration-like values described by doc can not be enforced

[ehennestad]

Some attributes / datasets have a doc specification listing a set of "allowed" values when dtype: "text"

Examples
ImageSeries.format (dataset)

nwb-schema/core/nwb.image.yaml

Lines 105 to 111 in 764aa37

	- name: format
	dtype: text
	default_value: raw
	doc: Format of image. If this is 'external', then the attribute 'external_file'
	contains the path information to the image files. If this is 'raw', then the raw
	(single-channel) binary data is stored in the 'data' dataset. If this attribute
	is not present, then the default format='raw' case is assumed.

Timeseries.data.continuity (attribute)

nwb-schema/core/nwb.base.yaml

Lines 182 to 190 in 764aa37

	- name: continuity
	dtype: text
	doc: Optionally describe the continuity of the data. Can be "continuous", "instantaneous", or
	"step". For example, a voltage trace would be "continuous", because samples
	are recorded from a continuous process. An array of lick times would be "instantaneous",
	because the data represents distinct moments in time. Times of image presentations would be
	"step" because the picture remains the same until the next timepoint. This field is optional,
	but is useful in providing information about the underlying data. It may inform the way this
	data is interpreted, the way it is visualized, and what analysis methods are applicable.

I did a quick search and could not find any related issues, but I am wondering if it was ever disussed to add something like an enumeration key. So for example, the specification for ImageSeries.format could look like this:

  - name: format
    dtype: text
    default_value: raw
    enum: external, raw
    doc: Format of image. If this is 'external', then the attribute 'external_file'
      contains the path information to the image files. If this is 'raw', then the raw
      (single-channel) binary data is stored in the 'data' dataset. If this attribute
      is not present, then the default format='raw' case is assumed.

[rly]

As discussed, I opened hdmf-dev/hdmf-schema-language#47 to address this.