D2b-asl

- Add `nibabel` as a dependency
- Add `"discard"` as an allowable item value in the `aslContext` array in `d2b-config.json`.
- Rename class `AslContext` -> `Aslcontext`
- Instantiating `Aslcontext` now accepts a list of strings and an optional file path, and no longer accepts an `Acquisition` object (for this see the `Aslcontext.from_acquisition()` classmethod)
- Asl acquisitions are now modified if non-BIDS-compliant labels are found. Specifically, if `"discard"` is a label, then the associated volume is removed from the acquisition.
- Introduce custom exceptions `MissingAslcontextError`, `InvalidAslcontextLabelError`, and `AslContextConfigurationError`
- Move aslcontext validation to `generate_context_file()` function.
- Add check to make sure that the number of labels == the number of volumes in the matching acquisition. (An `AslContextConfigurationError` is raised if there is a mismatch)

Optional *_aslcontext.json files
- Make the creation of *_aslcontext.json files optional.
- This is facilitated by the addition of two (mutually exclusive) flags to the `d2b run` command (specifically, `--[no-]include-aslcontext-json`). The "no" flag is taken to be the default if neither is specified, that is, no *_aslcontext.json files will be produced, regardless of whether of not asl data is found in the dataset.
- **NOTE:** These JSON files seem to cause errors in the bids-validator (v1.8.0), however, these files are listed in the spec, is this a bug in the validator?

First Release