improve test coverage, cleanup lint

Improve auto-linter, resolve minor issues thereby discovered

which: streamline logic

allow fpath to be a scalar or array

[h5,nc]save: allow bare filename

[h5,nc]save: allow plain filename without folder

ci: update actions

HDF5 scalar write, Matlab >= R2020b

Matlab doesn't have a high-level function to write scalar HDF5 dataset, so we added it to our h5save.

To avoid excess code, we put enhanced argument checks in stdlib.hdf5nc functions that require Matlab >= R2020b.

add fileio.sha256sum

v4.0.1

add fileio.sha256sum

reorganized as stdlib

combined a few complimentary libraries into this stdlib.

import hdf5nc.* => import stdlib.hdf5nc.*

bugfix: h5write size type

Some functions may pass a non-double size to h5write, which matlab needs, despite the data inherently being integer.
This casts size to double.

emptyness passthrough

the non-writing functions now return empty-like values when empty filename or other parameters are passed in. This seems to be a better match to expected behavior of other Matlab functions: empty in, empty out.

• {h5,nc}exist
• {h5,nc}size
• {h5,nc}variables

Separately, we moved CI to Azure since Travis-CI now has quotas, even for open-source projects.

add numerous tests

go to Class-based unit tests and use parameterization to find more corner cases

add lint tests

make internal functions private

this helps in reusing this code in external projects