Remove title from colorbar

[dnerini]

So far we indicated the units of the colorbar as its title. However, this keeps overlapping with the colorbar itself. The easiest fix is to remove the title and add the units to the label instead.

[codecov]

Codecov Report

Merging #260 (7fe6e2a) into master (10540d2) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #260      +/-   ##
==========================================
- Coverage   80.92%   80.92%   -0.01%     
==========================================
  Files         142      142              
  Lines       10760    10758       -2     
==========================================
- Hits         8708     8706       -2     
  Misses       2052     2052              

Flag	Coverage Δ
unit_tests	80.92% <100.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
pysteps/visualization/precipfields.py	82.70% <100.00%> (-0.26%)	⬇️

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