Inconsistencies between sympy.Dummy and symengine.Dummy

[bjodah]

Reporting this in order not to loose track of this. Noticed when trying to port some sympy dependent code to use symengine:

>>> import sympy as sy
>>> import symengine as se
>>> sy.Dummy('foo').name
'foo'
>>> se.Dummy('foo').name
'_foo'

I will probably work around this for the time being, but hopefully I'll get around writing a small PR soon.

>>> sy.ccode(sy.Dummy('foo'))
'foo_7783346'
>>> se.ccode(se.Dummy('foo'))
'_foo'