Fix implicit gmp to double conversion failures

[pentacular]

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Summary of Changes

Trivial fix for errors in compiling isotropic remeshing using gmp which fails to convert implicitly to double.

Release Management

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• Issue(s) solved (if any): fix #0000, fix #0000,...
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[janetournois]

@sloriot would it make sense to add a test that checks that isotropic remeshing compiles with GMP?

[sloriot]

do you think it would work to have double -> FT? From what I remember we said it would not be a good idea. Note also that with a specific vertex point map (on-the-fly conversion), this patch would not be needed.