Control zero humidity in thermo computations

[sandorkertesz]

Fixes #20

This PR adds the eps and out keyword arguments to

• dewpoint_from_specific_humidity
• dewpoint_from_relative_humidity
• temperature_from_saturation_vapour_pressure

with the follwoing meaning (e.g. for dewpoint_from_specific_humidity):

def dewpoint_from_specific_humidity(q, p, eps=1e-8, out=None):
...
eps: number
        If ``out`` is not None, return ``out`` when ``q`` < ``eps``.
        If out is None, ``eps`` is ignored and return np.nan for ``q``
        values very close to zero.
out: number or None
        If not None, return ``out`` when ``q`` < ``eps``. If None, ``eps`` is
        ignored and return np.nan for ``q`` values very close to zero.

Examples:

>>> from earthkit.meteo.thermo import dewpoint_from_specific_humidity
>>> dewpoint_from_specific_humidity(0., 10000.)
np.nan
>>> dewpoint_from_specific_humidity(0., 10000, out=100)
100
>>> dewpoint_from_specific_humidity(1e-6, 10000, eps=1e-5, out=100)
100

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.40%. Comparing base (d6175a8) to head (67d9211).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #23      +/-   ##
===========================================
+ Coverage    99.38%   99.40%   +0.02%     
===========================================
  Files            8        8              
  Lines          647      672      +25     
  Branches        20       26       +6     
===========================================
+ Hits           643      668      +25     
  Misses           4        4              

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[iainrussell]

Looks like a nice mechanism! I wonder if the kwargs need to be a bit more descriptive, such as zero_q_eps and zero_q_out_value - what do you think @sandorkertesz?

[sandorkertesz]

@iainrussell, I would like to keep it simple. Other, methods already use eps, while out is coming from numpy methods, e.g. np.log.