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
Classification for system of odes
Amendments on checking of linearity
Check for system of eq being solvable under classify_sysode

This PR contain methods to classify whether a given system of equation
is solvable by current solver. It is broadly divided on the basis of
parameters returned by classify_sysode which are linearity, no of eq
and order. If the return parameter by these methods is any "type" then
the given system is solvable otherwise None is returned which means
it is not solvable by current solving engine.
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.
4873: fix link to geometry plotting