SCHEMA: Add data array accessor to context

[effigies]

This is a formality, as nobody is actually dynamically building a context from schema.meta.context, but we have a couple rules that require access to the data array shape:

bids-specification/src/schema/rules/checks/dwi.yaml

Lines 20 to 42 in fc98780

	# 30
	DWIBvalRows:
	issue:
	code: BVAL_MULTIPLE_ROWS
	message: |
	'.bval' files should contain exactly one row of values.
	level: error
	selectors:
	- extension == ".bval"
	checks:
	- data.n_rows == 1
	# 31
	DWIBvecRows:
	issue:
	code: BVEC_NUMBER_ROWS
	message: |
	'.bvec' files should contain exactly three rows of values.
	level: error
	selectors:
	- extension == ".bvec"
	checks:
	- data.n_rows == 3

I have implemented this in the JS validator as part of https://github.com/bids-standard/bids-validator/pull/1797.

It is not future-proof, but we reserve the option to make breaking changes to the schema.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (fc98780) 87.83% compared to head (ffe28ff) 87.83%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1609   +/-   ##
=======================================
  Coverage   87.83%   87.83%           
=======================================
  Files          16       16           
  Lines        1356     1356           
=======================================
  Hits         1191     1191           
  Misses        165      165           

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[effigies]

Superseded by #1622.