Pyjams

* Add `gridcellarea` to calculate the area of grid cells on Earth in
square metre.
* Add `kernel_regression` and `kernel_regression_h` for multi-dimensional
non-parametric kernel regression.

* Add `infonetcdf` and `readnetcdf` to get variables from or print
information of a netcdf file.
* Add `get_color` to get value of named colors known to Matplotlib.
* Added named colors of the guidelines of the Helmholtz Centre for
Environmental Research - UFZ, Leipzig, Germany.

* Use `array2input` and `input2array` in `division`, correcting bug with
scalar input and getting rid of numpy geterr and seterr.
* Enhanced `array2input` to take second input variable.
* Added functions `isundef` in `helper` module to deal with NaN and Inf.
* Added 'order' keyword to `get_cmap`.
* Added functions `array2input` and `input2array` in new `helper` module to
assure same input and output types. Use them in `esat` and
`alpha_equ_h2o`. The `helper` module is not in `__init__.py` nor in the
documentation (yet?).
* Return numpy array if type(input)(output) fails for unknown iterable
types in `esat` and `alpha_equ_h2o`.

* Always close open files in module `fsread`.
* Set default fill_value to NaN for floats in module `fsread`.
* Remove read_only mode for openpyxl in `xread` because closing is disabled
in this case.
* Change handling of return type to allow more (unspecific) iterable types
such as pandas time series in `esat` and `alpha_equ_h2o`.
* Added `xread`, `xlsread`, and `xlsxread`, reading numbers and strings
from an Excel file into 2D float and string arrays.

* Added `mad`, median absolute deviation test.

* Added `esat`, giving saturation vapour pressure over water and ice.
* Bug in `alpha_equ_h2o` in return type if list or tuple and undef.