expression series expansion

The first code for series expansion was a translation of SymPy ring series (the univariate case) using Piranha polynomials with rational coefficients. This has been improved to handle symbolic constants using Piranha polynomials over Expressions so we have now:

• series.h series() interface and generic implementation
• series.cpp Interface implementation
• series_visitor.h Link series to expression tree
• series_piranha.h, series_piranha.cpp Specific implementations: URatPSeriesPiranha and UPSeriesPiranha
• series_flint.h, series_flint.cppURatPSeriesFlint very fast univariate implementation
• heuristics in series() to chose between (at the moment) UPSeriesPiranha and URatPSeriesFlint for optimal result/speed

In progress:

• Python/SymPy interface---this was first slowing down our good performance because of https://github.com/sympy/sympy/issues/10262 which should be resolved by https://github.com/sympy/sympy/pull/10513 and https://github.com/symengine/symengine.py/issues/48

What's immediately missing from current series IMHO:

• more functions (bessel, gamma, zeta...)

  NOTE : This is handled by #813, but poles need to be special cased and handled like gamma.

• implement Taylor series starting point parameter x0

• continuation of previous: expansion at oo, asymptotics with oo. Example:

>>> y = series(1 / (x+1), x, oo, 3)
>>> str(y)
'x**(-3) - 1/x**2 + 1/x + O(x**(-4), (x, oo))'

• full multivariate expressions (not sure, maybe 1/(1-x-x*y) works already?) Pari:

? Ser(1/(1-x-x*y)) 
%2 = 1 + (y + 1)*x + (y^2 + 2*y + 1)*x^2 + (y^3 + 3*y^2 + 3*y + 1)*x^3 + ...

• SymEngine implementation of UPSeriesPiranha with no additional dependencies. (Modify series_generic.h to fit into series.h model)

  NOTE : currently being handled here