Invert quadrature loops (#322)

Invert the quadrature loop in the assembly methods of linear and
bilinear forms, which means that we loop over the quadrature points
before looping over the basis functions. This loop inversion is not
automatic, we do it only in the case where we have a spline or NURBs
mapping, or where the analytical mapping is not affine.

To manually deactivate the loop inversion it is sufficient to set
`invert_quad_loop = False` in function `AST.__init__` of module
`psydac.api.ast.fem`.

**Other changes**

- Add `lhs` property to class `ComputeLogicalBasis`.
- Delete `prefix` property from classes `BasisAtom`, `IteratorBase`,
 `LogicalBasisValue`, and `Loop`.
- Delete initialization code from `_visit_EvalField` and `_visit_EvalMapping`,
 and move it to the constructors of the classes `EvalField` and `EvalMapping`.