Tutorial for Finite Difference of Derivatives
Ran stripwhitespace on finite_diff_derivatives.rst given recent merge

This is a commit of the file after running stripwhitespace, given Chris Smith's
recent enabling of usage of stripwhitespace compatibility with .rst files.
Fix of some further typos
Fix of some suggested typos and code changes
Some fixes of typos...
Stripped whitespace again as a check
Some more prose changes
Remove whitespace and add some commas
Made new directory special_topics, moved finite_diff_derivatives.rst to special_topics dir, etc

After some discussion I have made a new subdirectory src/special_topics to contain longer and perhaps not at tutorial
level.  I also put src/special_topics/finite_diff_derivatives.rst,a new index.rst and a new intro.rst in the new
directory. I have edited src/tutorial/index.rst to reflect the content change.  And I modified the src/index.rst
to contain a reference to SymPy Special Topics.
Further small edits and clarifications
Changed print to print() and xrange to range for compatibilty with Python 3
Removed the currently empty section header on Forberg's method for derivatives
Fixed some problems with using print() and pprint.
Further attempts to fix problems with using print, pprint solved.
Cleaned up usage of print, removed pprint. Now hopefully done.
Repaired missing outputs of print() statements. Now all examples run correctly.
modify/correct Integral.is_number

isyms should start with the free symbols of the integrand, not all
symbols.

A 2nd non-heroic test for a case where a free symbol will not
affect evaluation was added to is_number and to evalf for integrals.
If the variable of integration does not appear in the free symbols
then only the difference in the limits is of significance, e.g.

Integral(x, (y, d, d + 1)) will introduce a linear factor of y
and the value of d is not significant. This integral is the same
as Integral(x, (y, 0, 1)).
add additional is_number test for Integral

[skip ci]
Merge pull request #7521 from smichr/n

modify/correct Integral.is_number
address indeterminacy in FiniteSet