Bijker spline

See issue #880 . Spline interplation insetad of linear interpolation in the calculation of einstein integrals.

added module:gaia label

assigned to @sara.pavan

added 21 commits

• ba5f9602...43198866 - 19 commits from branch main
• ee786308 - evaluate einstein integrals using cubic spline
• 95fe322e - [VnV][gaia] Update of ref file

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added 1 commit

• fada148e - bugfix eistein integral. forgotten save statement leading to weird results

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added 1 commit

• f39f0e02 - [src][gaia] Move variables with SAVE statements

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added 1 commit

• 0a4f8a59 - modified routine get_eindata_gaia.f to make sure in compiles on the intel compiler

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It is suggested to replace the new subroutine get_eindata_gaia by an ASCII file containing the same values to be read.
Actual configuration is not compiling with intel due to the too many continuation lines.