Bugfix in slab geometry when using Lconstraint=3, and output of stability variables

[ErolBa]

This pull request involves the following modifications:

1. Primary modification: fixes a bug when running with Igeometry=1 and Lconstraint=3, adds the total pflux as a conserved quantity (otherwise the Beltrami solver is missing a conserved quantity).
2. Outputs .hessian matrix for stability analysis
3. Calculates and outputs plasma beta
4. Outputs quantities related to stability in the .h5 file (eigvals, eigvecs, and the hessian matrix)
5. Adds a test for G=1 Lcons=3

[smiet]

Just a few changes here and there, see the review