Some fixes of typos...
Stripped whitespace again as a check
Removed locks and some if statements to improve performance
Some more prose changes
Remove whitespace and add some commas
add variable size to lru_cache through environment variable SYMPY_CACHE_SIZE
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.
Merge pull request #7419 from amitsaha/plot_implicit

plot_implicit: Explicitly specify x and y
XXX leave experimental_lambdify test unchanged
remove tabs from files
cache_debug now appropriately handles SYMPY_CACHE_SIZE
Various items listed as autoclass which should be autofunction
suppress text plots
Merge pull request #2882 from rajaths589/addSteradian

Add steradian unit: solid angle
remove all collinear points
Merge pull request #7479 from smichr/geom

remove all collinear points
Add wraps decorator
Catch ZeroDivisionError in is_constant
Merge pull request #7474 from debugger22/suppress_plot

Suppress plots in the tests
Merge pull request #2905 from avichaldayal/3411_fresnel

3411 fresnel