Fix wavelength range print out to use regular nbsp

[mraspaud]

The International System of Units page on wikipedia says:

The value of a quantity is written as a number followed by a space (representing a multiplication sign) and a unit symbol; e.g., 2.21 kg, 7.3×102 m2, 22 K. This rule explicitly includes the percent sign (%)[28]:134 and the symbol for degrees Celsius (°C).[28]:133 Exceptions are the symbols for plane angular degrees, minutes, and seconds (°, ′, and ″, respectively), which are placed immediately after the number with no intervening space.

(https://en.wikipedia.org/wiki/International_System_of_Units#General_rules)

so a regular (not narrow) space is now used when displaying wavelength ranges.

• Closes Encoding of wavelength range #1449
• Tests added

[codecov]

Codecov Report

Merging #1466 (029b5ee) into master (c0e21e6) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1466   +/-   ##
=======================================
  Coverage   90.58%   90.58%           
=======================================
  Files         239      239           
  Lines       34313    34313           
=======================================
  Hits        31081    31081           
  Misses       3232     3232           

Flag	Coverage Δ
behaviourtests	4.53% <0.00%> (ø)
unittests	91.05% <100.00%> (ø)

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Impacted Files	Coverage Δ
satpy/dataset/dataid.py	93.71% <100.00%> (ø)
satpy/tests/test_dataset.py	100.00% <100.00%> (ø)

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[djhoese]

What does @gerritholl have to say about this? Wasn't there some ISO specification for non-breaking spaces and units?

[sfinkens]

LGTM, thanks a lot!

[mraspaud]

@sfinkens did you get to test it in your sensitive terminal?

[sfinkens]

Yes it works fine!

[gerritholl]

I thought this was in ISO 80000-1, but apparently not.

§7.1.4 states:

The symbol of the unit shall be placed after the numerical value in the expression for a quantity, leaving a space between the numerical value and the unit symbol. It should be noted that this rule also applies to the units per cent, %, and per mil, ‰. It should also be noted that, in accordance with this rule, the symbol ℃ for the degree Celsius shall be preceded by a space when expressing a Celsius temperature.

The only exceptions to this rule are for the units degree, minute and second for plane angle, in which case there shall be no space between the numerical value and the unit symbol.

Then there is §7.2.2 about compound units, which states that a compound unit may be written as "N · m", "N m", or "Nm". The latter alternative is not permitted if the first unit is either metre or Tesla as those can be confused with prefixes milli and tera, respectively.

The ISO standard doesn't specify what type of space shall be placed between the value and the symbol. Since we're only handling wavelengths here and not angles, the display was fine with ISO 80000-1 before this PR and remains fine with ISO 80000-1 after this PR.