Make Airy functions symbolical

[olazo]

As discussed in ​sage-support.

Currently sage can evaluate airy functions numerically:

sage: airy_ai(1.4)
0.0820380498076

but it doesn't work symbolically

sage: airy_ai(x)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)

/home/oscar/Escritorio/tesis/calculos/<ipython console> in <module>()

/home/oscar/sage/sage-4.7.1/local/lib/python2.6/site-packages/sage/functions/special.pyc in airy_ai(x)
    621    """
    622    _init()
--> 623    return RDF(meval("airy_ai(%s)"%RDF(x)))
    624 
    625 def airy_bi(x):

/home/oscar/sage/sage-4.7.1/local/lib/python2.6/site-packages/sage/structure/parent.so in sage.structure.parent.Parent.__call__ (sage/structure/parent.c:7102)()

/home/oscar/sage/sage-4.7.1/local/lib/python2.6/site-packages/sage/structure/coerce_maps.so in sage.structure.coerce_maps.NamedConvertMap._call_ (sage/structure/coerce_maps.c:4221)()

/home/oscar/sage/sage-4.7.1/local/lib/python2.6/site-packages/sage/symbolic/expression.so in sage.symbolic.expression.Expression._real_double_ (sage/symbolic/expression.cpp:5391)()

/home/oscar/sage/sage-4.7.1/local/lib/python2.6/site-packages/sage/symbolic/expression.so in sage.symbolic.expression.Expression._eval_self (sage/symbolic/expression.cpp:4898)()

TypeError: Cannot evaluate symbolic expression to a numeric value.

We should make it symbolical for both airy_ai and airy_bi, as well as their derivatives.