Pyjams

* Added functions `eair2rhair`, `rhair2eair`, `eair2vpd`, `vpd2eair`,
`rhair2vpd`, `vpd2rhair`, `eair2shair`, `shair2eair`, `eair2mrair`,
`mrair2eair`, for conversions between partial pressure, relative humidity,
and specific humidity of water vapour in air.
* Renamed module `esat` to `air_humidity`.
* Correct treating of undef if two arrays given in `array2input` helper
function.
* Updated all constants related to gases in `const` module for redefinition
of SI units of 2019.
* Renamed some constants in `const` for greater consistency.
* Keyword only_use_pyjams_datetimes in `num2date` to be able to use all
implemented methods of `datetime` class.
* Allow also CF-calendars in `datetime` class.
* Updated tests for `sce`.

* Add `updatez` and `updatez_compressed` to update arrays in a single file
in numpy's npz format.
* Do not set color for missing data; only existed for sron palettes.
* Add --dpi as a standard option in `mcPlot`.
* Use mpl.colormaps[name] instead of mpl.colormaps.get_cmap(name)
to replace mpl.cm.get_cmap(name) to work with matplotlib < v3.6.

* Assure 4-digit years and catch %04Y format errors on Windows in
`date2date` and in datetime class.
* Use assert_almost_equal for fractional days with microseconds in tests of
datetime class
* Changed matplotlib.cm.get_cmap to matplotlib.colormaps.get_cmap in
color module and tests.
* Skip test for float128 on Windows in `ncio`.
* Python 3.6 might not be fully supported anymore.
* Remove dependency to ``partialwrap`` for tests.
* Adding `sce` the Shuffled-Complex-Evolution algorithm for function
minimization.
* Adding 'a wide variety of' to ``pyjams`` key phrase.
* NA to NaN, i.e. R to Python convention in `fsread`.

* round_microseconds method for `datetime` class if dates are from
non-microsecond precise origin.
* Added return_arrays keyword in `date2num`.
* calendar keyword takes precedence on calendar attribute of
datetime objects in `date2num`.
* Add left, bottom, top to standard layout options in `mcPlot`.
* Documented as_cmap keyword of `get_cmap`.
* Added `means` to calculate daily, monthly, yearly, etc. means.
* Allow scalar input in `num2date`.
* Renamed `datetime.py` to `class_datetime.py`.

* Allow negative dates in `date2date`.
* Support of microseconds in `date2date`.
* Assure that `input2array` gives no 0d-array.
* More `datetime` tests and bug fixes for microseconds and has_year_zero.
* Delete unnecessary HDF5 filters in variable definition in `netcdfio`
for compatibility with netcdf4 > 1.6.0.
* Reduce precision to 14 digits for tests of `logistic2_offset`.
* Remove Python version 3.7 from CI but leave version 3.6 because of
problems with netcdf4 for 3.7 on Windows.

* Removed documentation from `Read the Docs` and moved to Github Pages:
https://mcuntz.github.io/pyjams/
* More `datetime` tests and bug fixes.