Update polar spline test plots

• Allow interactive changing of spline elements and degree.
• 1-form and 2-form test functions and their curl and div, respectively.
• Implement plotting of discrete curl and div in polar spline test.
• Fix analytical curl, and draw origin (pole).
• Add two test functions f(x,y,z) = sin(x) and f(x,y,z) = x.
• Partial fix of contour plot coloring issues.
• Implement mapping 'hollow_ellipse'.
• Implement mapping 'rotated_ellipse'.
• Implement various F maps.
• Fix pyccel compilation. Implement 'soloviev_approx' mapping.
  • Because Fortran is case insensitive, we cannot use the same variable name with different cases in the same function (e.g. Lz and lz). Even though they are local variables inside if-else statements, their scope is "global" (visible) to the whole function.
• Capitalize lx to Lx in mappings_3d. Update doc for 'hollow_cyl'.
• Fix one missing replacement of 'hollow_ellipse' with 'ellipse'.
• Zoom plot to pole.
• Implement modified Soloviev map with sqrt dependence on eta1.