Removed redundant part of code in compilef.
Replace free_symbols over atoms
merge conflict resolution
remove str from a test file added
Merge branch 'master' into clear-atoms(Symbol)
merging master with newly updated files
modify ode as to eliminate redundant lines
remove _find_first_symbol as no use
changing constructor as to support Integral and Sum forms
reverted back the constructor.py by removing simplification codes
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.
Catch ZeroDivisionError in is_constant