Move custom rf class outside the combsimp definition

This fixes, or rather, works around, a bizarre caching issue involving classes
defined inside functions.  See the discussion on pull request #1710.
Fixes #2676 : Convert Matrix to NumPy Array with NumPy dtype

Exposes dtype to the user. Default dtype is 'object'.
Rename d_type to dtype
Added tests for matrix2numpy
Changed tol on Complex and Real field to None

Fixes simplification issue #7263. Added test to check issue in
test_simplify.
Fixed travis errors

Direct comparison of expressions lacks enough precision for ==.
Comparing numerically now with evalf().
Adds Derivative.as_finite_diff() and core.finite_diff with finite_diff_weights and apply_finite_difference
Change default step of as_finite_diff from sympify('h') to 1
Fixes Py3 error for test range+range
Moved finite_diff from core to series. Extended doc of finite_diff.
Updated Derivative.as_finite_diff() doc, tests and default step-size.
Merge remote-tracking branch 'upstream/master' into finite_difference
Fixed/updated docstrings with new master
Fixed docstring.
Added non homogeneoous linear ruler eq
Corrected the documentation
corrected nameerror
Added trialset match in nonhomogeneous euler eq
Fixed the error coming due to Dummy
added simplfication of log function