ipython_view: Fix off-by-one number in input prompt
ipython_view: Fix off-by-one number in input prompt

Previously, the numbering for the input prompt
was incorrect, one too low except for the
first prompt, so there was a mismatch between
the numbers for input and output: An "In [n]"
would be followed by an "Out [n+1]", e.g.

    In [1]: 1
    Out[1]: 1
    In [1]: 2
    Out[2]: 2
    In [2]: 3
    Out[3]: 3
    In [3]: _i2 # should give the input with index 2
    Out[4]: '2'
    In [4]:

Fix this by generating the next input prompt
*after* the current input has been processed
(`self.IP.run_cell(source_raw, store_history=True)`),
at which point the counter (`self.IP.execution_count`)
has the correct value:

    In [1]: 1
    Out[1]: 1
    In [2]: 2
    Out[2]: 2
    In [3]: 3
    Out[3]: 3
    In [4]: _i2
    Out[4]: '2'
    In [5]:

GNOME/accerciser,GNOME/accerciser