Add precondition check for SortHead

[baziotis]

The transformation <anything>.sort_values().head() --> <anything>.nsmallest(n=5) is correct only if <anything> is a pandas.Series (it also works for DataFrames but we don't support it for now). Currently, we don't perform this check.

In the following example, Dias incorrectly rewrites code:

import pandas as pd
import dias.rewriter
class Foo:
    def sort_values(self):
        return self
    def head(self):
        return self
----- NEW CELL ----
%%rewrite
foo = Foo()
foo.sort_values().head()

Add the check into the rewritten code. So, if the original code is res = <anything>.sort_values().head(), the rewritten code should be:

_REWR_tmp0 = <anything>
if isinstance(_REWR_tmp):
  _REWR_tmp1 = _REWR_tmp0.nsmallest(n=5)
else:
  _REWR_tmp1 = _REWR_tmp0.sort_values().head()
res = _REWR_tmp1