fix two aspects of latex printing: absolute values and imaginary unit

\imath is almost never what you want when rendering the imaginary unit
to latex. its purpose is so that you can put things like bar, vec, hat,
dot, tilde, etc. onto the character, none of which really make sense
for i. bar (representing complex conjugation) makes some sense, but
\bar{i} equals -i by definition, so this is a rare case.


enter the following into live.sympy.org to see an illustration of both fixes:

i = sqrt(-1)
Abs(zeta(Integer(1)/2 + i * t))