Fix formatting of NMR position labels

[lcnittl]

e8e88c9 in its attempt to fix #24 introduced a bug, namely that it prefixed the argument in

chemmacros/code/chemmacros.spectroscopy.code.tex

Line 367 in 05e767a

\exp_not:n { _{#1} }

with an underscore to simulate sub mode of pos-number.

Indeed, the actual problem was, that OP of #34 wanted to use sub without specifying this in the spectroscopy module's option. The "default" value of pos-number, however, is side.

A bug in

chemmacros/code/chemmacros.spectroscopy.code.tex

Line 161 in 05e767a

pos-number / .initial:n = side ,

(superfluous /) caused the spectroscopy module to behave weirdly, producing a mix of side and sub. The supposed fix in e8e88c9 breaks the spectroscopy module when specifying both sub or super as it produces an underscore character (sub- or superscripted, respectively) followed by standard formatted text in these cases, while producing sub style when not specifying any pos-number.