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
merge with master
simplify Integral.is_number routine
Merge branch 'master' into atoms
restore Integral.is_number routine
Merge pull request #7508 from smichr/atoms

use free_symbols instead of atoms(Symbol) where appropriate