gh-37823: Fix LaTeX output of FreeModuleTensor.display_comp
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This PR fixes an issue of the method `display_comp` of class
`FreeModuleTensor`, which arises when the LaTeX name of a tensor
contains the symbol `^` or `_`. For instance, in Sage 10.4.beta3, we
have:
```
sage: E.<x,y> = EuclideanSpace()
sage: g = E.metric()
sage: latex(g.inverse().display_comp())
\begin{array}{lcl} g^{-1}_{\phantom{\, x}\phantom{\, x}}^{ \, x \, x } &
= & 1 \\
g^{-1}_{\phantom{\, y}\phantom{\, y}}^{ \, y \, y } & = & 1 \end{array}
```
This is incorrect LaTeX syntax: the symbol `g` gets two exponent
markers (`^`). Consequently, in a Jupyter notebook in `%display latex`
mode, the command `g.inverse().display_comp()` will not generate any
typeset output.
This is fixed here by making the LaTeX output of `display_comp` more
robust: the LaTeX symbol of the tensor (`g^{-1}` in the above example)
is now enclosed between two braces (e.g. `g^{-1}` is changed into
`{g^{-1}}`) before adding indices to it.
URL: https://github.com/sagemath/sage/pull/37823
Reported by: Eric Gourgoulhon
Reviewer(s): Matthias Köppe
