Add expression interpreter

[ubdbra001]

Here's the initial attempt at adding all the parts of the expression language interpreter.

These successfully cover the full range of tests in the bidsschematools expression tests.
Two things that still need addressing:

• exists passes the tests but needs more work to actually work as described in the schema tools docs
• I'm not sure yet how to integrate them with the Context

[codecov]

Codecov Report

❌ Patch coverage is 20.27027% with 118 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.38%. Comparing base (1963ba3) to head (39d7085).

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #71       +/-   ##
===========================================
- Coverage   90.89%   80.38%   -10.52%     
===========================================
  Files          13       14        +1     
  Lines         846      994      +148     
  Branches      124      175       +51     
===========================================
+ Hits          769      799       +30     
- Misses         47      165      +118     
  Partials       30       30               

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[effigies]

From today's call, here's a quick-and-dirty proxy to allow you to provide multiple namespaces and treat them as one:

class LookupProxy:
    def __init__(self, *objs):
        self.objs = objs

    def __getitem__(self, key):
        for obj in self.objs:
            if isinstance(obj, dict) and key in obj:
                return obj[key]
            if hasattr(obj, key):
                return getattr(obj, key)
        raise AttributeError

    # Provide either attr or item lookup
    __getattr__ = __getitem__

Example usage:

import bids_validator as bv
import bids_validator.context
from bids_validator import expression_language as el
from bids_validator.types.files import FileTree
from bidsschematools.schema import load_schema

schema = load_schema()

root = FileTree.read_from_filesystem('tests/data/bids-examples/ds000117/')
ds = bv.context.Dataset(root, schema)

file = root / 'sub-01' / 'ses-mri' / 'anat' / 'sub-01_ses-mri_acq-mprage_T1w.nii.gz'
context = bv.context.Context(file, ds, None)

namespace = LookupProxy(el.ids, context)

[ubdbra001]

The LookupProxy class suggested works as is, but I had to make a couple of tweaks to the new_evaluator function to get it working with both the expression tests and actual data.
I had a go with parsing and evaluating some rules from the schema, and, of the limited subset I tried, they all produced the correct output.