Allow powers of SignGroup elements to live in other parents

[behackl]

A SignGroup is used as an auxiliary construct in the context of asymptotic expansions to split the growth of terms like (-2)^n * n^42 into, essentially, 2^n * n^42 * (-1)^n that lives in a cartesian product QQ^n * n^QQ * Signs^n.

In the current implementation of Sign.__pow__, the power is computed using the raw data (essentially, int ** whatever) and then an attempt is made to push the result back into SignGroup. This changes this behavior to still attempt the conversion -- but if it fails while the raw operation itself was successful, the result living in some other parent not coercing back to SignGroup is returned.

Simple example:

sage: from sage.groups.misc_gps.argument_groups import SignGroup
sage: S = SignGroup()
sage: x = SR.var('x')
sage: S(-1)^x  # this fails on develop
(-1)^x
sage: _.parent()
Symbolic Ring

For asymptotic expansions, this fixes an issue preventing substitutions in expansions with terms that involve, e.g., (-1)^n.

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[fchapoton]

why recompute the power ? and maybe use "return" in the try ?

[behackl]

You are absolutely right, I changed the code in haste and missed replacing the argument. Fixed via c431e2c.

[fchapoton]

ok, looks good enough, although maybe not very kosher in term of the coercion model