Extended subscripts are not supported

[aleiot]

The function LaTeX2Unicode.convert(String) does not support subscripts longer than 1 character in the LaTeX input. For example:
x_{n+1}
is converted to:
x_(n+1)
while:
x_n
is correctly converted to:
x_n

On the other hand, the same is not true for superscripts. Superscripts longer than 1 character in the LaTeX input are correctly converted to unicode.