Add various Maxima special functions to symbol table

[kcrisman]

We're missing some conversions from Maxima. Like exponential integrals of various kinds.

sage: f(x) = e^(-x) * log(x+1)
sage: uu = integral(f,x,0,oo)
sage: uu
x |--> e*expintegral_e(1, 1)

See ​this ask.sagemath post for some details.

sage: sage.symbolic.pynac.symbol_table['maxima']
{'elliptic_e': elliptic_e, 'imagpart': imag_part, 'acsch': arccsch, 'glaisher': glaisher, 'asinh': arcsinh, 'minf': -Infinity, 'elliptic_f': elliptic_f, '(1+sqrt(5))/2': golden_ratio, 'inf': +Infinity, 'log(2)': log2, 'kron_delta': kronecker_delta, 'asin': arcsin, 'log': log, 'atanh': arctanh, 'brun': brun, '%pi': pi, 'acosh': arccosh, 'sin': sin, 'mertens': mertens, 'ceiling': ceil, 'infinity': Infinity, 'elliptic_ec': elliptic_ec, 'atan': arctan, 'factorial': factorial, 'twinprime': twinprime, 'khinchin': khinchin, 'catalan': catalan, 'signum': sgn, 'binomial': binomial, 'delta': dirac_delta, 'asec': arcsec, 'elliptic_kc': elliptic_kc, '%gamma': euler_gamma, 'realpart': real_part, 'elliptic_eu': elliptic_eu, 'cos': cos, 'acoth': arccoth, 'gamma_incomplete': gamma, 'li[2]': dilog, 'atan2': arctan2, 'exp': exp, 'psi[0]': psi, 'asech': arcsech, 'acos': arccos, 'acot': arccot, 'acsc': arccsc, 'elliptic_pi': elliptic_pi}