F90nml

- Bugfix: patch() and update() now work with Cogroups. To specify a
particular cogroup, use the `_index` meta key in the patch. Otherwise,
the patch is applied to all members of the cogroup.
Thanks to mandresm and seb-wahl for reporting and collaborating on the
design.
- Multidimensional negative index support
- patch no longer pops off values applied to the namelist
Thanks to Armin Corbin and Claire Carouge for reporting.
- Improved NumPy (and xarray) compatibility through introduction
Thanks to Liam Pattinson for contributing this fix.

- Bugfix: Cogroup namelist initialization from a cogroup was fixed.
Thanks to Rasmus Fogh for reporting.

- Bugfix: Case-insensitive del now works as intended.
Thanks to GitHub user mandresm for reporting.

- Bugfix: Several cogroup issues related to management of internal keys and
todict() output were fixed. Thanks to GitHub user poluhh and Rasmus H.
Fogh for reporting.
- Bugfix: Empty lists [] are now displayed as comma-delimited blanks.
Thanks to William Dawson for reporting.

- Ordering of duplicate keys is now preserved, through the introduction of
two new classes. The Cogroup extends the list previously associated with
the values of a duplicate key. The NmlKey extends str and externally
holds the namelist key, but also contains an internal key to distinguish
between duplicates.
- A repeated key is no longer initialized with a list, and must now be
explicitly declared as a cogroup with the `add_cogroup` function.
- A performance issue related to the detection of values in a line has been
fixed. Thanks to Stephen LeDoux for reporting and Philip Cross for
fixing this issue.

- String splitting is now an optional feature, enabled by the split_strings
property. Default string array output again resembles v1.2 output.
- Bugfix: For arrays of strings, a split string element will no longer
insert erroneous commas. Thanks to Jared Lewis for reporting.