Adding metric terms to account for non-rectangular shape of grid cells

This PR corrects an implementation of a finite-element solver for ice-sheet/shelf velocity. It computes a Jacobian weight J_q in shape functions for quadrature points and applies it to integrals over cells. This correction is significant when the shape of the cell is not rectangular. The same corrections are applied for subgrid parameterizations.

Additionally, it fixes missing deallocations of several arrays.

Changes made:
 - nitialize_ice_shelf_dyn: allocates a new variable `Jac`
 - bilinear_shape_fn_grid: computes a Jacobian determinant (`Jac`) at each quadrature point
 - CG_action: uses `jac_wt= Jac * IareaT` for computing the ice deformation and basal traction terms
 - matrix_diagonal: uses `jac_wt= Jac * IareaT` for computing diagonal terms
 - CG_action_subgrid_basal: computes `jac_sub_wt` using new arguments `dxCv_S`, `dxCv_N`,`dyCv_E` and `dyCv_W` that determine cell-edge spacing
 - ice_shelf_dyn_end: deallocates `Jac`, `CS%sx_shelf`, `CS%sy_shelf`, `CS%u_flux_bndry_val`, `CS%v_flux_bndry_val`,`CS%calv_mask`, `CS%Phi`, `CS%Phisub`, `CS%PhiC`.

Copilot (model Claude Sonet 4.6) assisted with this PR.

Author: OlgaSergienko