cpdfcrop : crop pdf figures to their minimal size 
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Remove empty margins, i.e. extra white space, of an input PDF
figure. The resulting output figure occupies the minimal paper size
needed for the content and is therefore suitable for inclusion as a
graphic.   

**References** : http://manpages.ubuntu.com/manpages/precise/man1/pdfcrop.1.html

**Provider / contact** : climaf at meteo dot fr

**Inputs** : any figure object (but only one)

**Mandatory arguments**: none

**Optional arguments**:
  - none

**Output** : the PDF cropped figure 

**Climaf call example**::
 
  >>> cdef("frequency","monthly")
  >>> cdef("period","198001")
  >>> tas=ds(project="example", simulation="AMIPV6ALB2G", variable="tas") 
  >>> fig=plot(tas,title="title",format="pdf")
  >>> cshow(fig)
  >>> cropped_fig=cpdfcrop(fig) # some figure object
  >>> cshow(cropped_fig)

**Side effects** : none

**Implementation** : standard 'pdfcrop' utility to remove empty
margins