Fix combine_metadata not handling lists of different sizes

[djhoese]

The combine_metadata utility function doesn't seem to handle lists of different sizes. Before this PR you'd get something like this:

~/repos/git/satpy/satpy/dataset/metadata.py in _all_values_equal(values)
    179 def _all_values_equal(values):
    180     try:
--> 181         return _pairwise_all(nan_allclose, values)
    182     except TypeError:
    183         return _pairwise_all(eq, values)

~/repos/git/satpy/satpy/dataset/metadata.py in _pairwise_all(func, values)
    144 def _pairwise_all(func, values):
    145     for value in values[1:]:
--> 146         if not func(values[0], value):
    147             return False
    148     return True

<__array_function__ internals> in allclose(*args, **kwargs)

~/miniconda3/envs/satpy_py37/lib/python3.7/site-packages/numpy/core/numeric.py in allclose(a, b, rtol, atol, equal_nan)
   2254 
   2255     """
-> 2256     res = all(isclose(a, b, rtol=rtol, atol=atol, equal_nan=equal_nan))
   2257     return bool(res)
   2258 

<__array_function__ internals> in isclose(*args, **kwargs)

~/miniconda3/envs/satpy_py37/lib/python3.7/site-packages/numpy/core/numeric.py in isclose(a, b, rtol, atol, equal_nan)
   2363     yfin = isfinite(y)
   2364     if all(xfin) and all(yfin):
-> 2365         return within_tol(x, y, atol, rtol)
   2366     else:
   2367         finite = xfin & yfin

~/miniconda3/envs/satpy_py37/lib/python3.7/site-packages/numpy/core/numeric.py in within_tol(x, y, atol, rtol)
   2344     def within_tol(x, y, atol, rtol):
   2345         with errstate(invalid='ignore'):
-> 2346             return less_equal(abs(x-y), atol + rtol * abs(y))
   2347 
   2348     x = asanyarray(a)

ValueError: operands could not be broadcast together with shapes (4,) (0,) 

This PR fixes this by catching this ValueError when trying to compare lists. I wanted to have a simple additional check like if len(x) != len(y): return False, but the function that is triggering this issue is meant to compare any value including those that may not have a len parameter.

• Closes Can't resample GOES Meso data when using night IR composite #1632
• Tests added

[codecov]

Codecov Report

Merging #1643 (c3e6f0c) into master (7aa706b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1643   +/-   ##
=======================================
  Coverage   92.62%   92.62%           
=======================================
  Files         258      258           
  Lines       37810    37828   +18     
=======================================
+ Hits        35022    35040   +18     
  Misses       2788     2788           

Flag	Coverage Δ
behaviourtests	4.81% <0.00%> (-0.01%)	⬇️
unittests	92.76% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
satpy/dataset/metadata.py	98.66% <100.00%> (ø)
satpy/tests/test_dataset.py	100.00% <100.00%> (ø)
satpy/readers/electrol_hrit.py	91.66% <0.00%> (ø)

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[mraspaud]

Looks good, just a couple styling/testing comments.

[mraspaud]

LGTM