- This version of select() should be more robust in dealing with
  parameters that exist in "top level" of metadata dict
- I hope it is also easier to understand

Some of this was done in another branch, and this one rebased

Pylint doesn't like the call to ._set_spectrum_data on another class
instance because it doesn't understand that this instance was just
created with the current class. I think that means the warning is ok
to suppress.

This adds toolz as a dependency to the main package. We don't
anticipate that causing a lot of problems; it is a small, stable,
pure-python library also available on conda-forge.

Linters hate the named lambdas. I think they are quite nice because
they are compact and immediately draw attention to the "one-liner"
they attach a name to...

but maybe the more explicit form with type hints will make it easier
for someone to understand in future.

This structure seems a bit more legible and should reduce redundancy
in Spectrum2DCollection

Collect the axis-twiddling code in one place to improve readability.

- Refactor re-used data import to use pytest fixture
- Remove initial from_spectra test; new one covers it all
- Test from_spectra with inconsistent input
- Test mixin-supplied methods

Make things a little cleaner and more idiomatic

Co-authored-by: Jacob Wilkins <[email protected]>

Suggested by @oerc0122

- This avoids some of the repetition in dict comprehensions to remove
  an element
- Here we also slightly rework _combine_metadata so it is clearer what
  each variable represents.

list comprehension is little clunky but avoids the 1-length special
case: cleaner overall

Via discussion / pair-programming with @oerc0122

- Use native dict comprehension over keyfilter in iter_metadata: it's
  a bit ugly but no more complicated, and should be easier to read
  "casually"

- Clearer comment re: value-pair combination

- Replace a lambda with named partial function and toolz complement

Co-authored-by: Jacob Wilkins <[email protected]>