Closes #198

Changes made to solve the bug: when selecting upper and lower bounds of the x and y coordinates, the sources were placed in the wrong position. Rotation of 90 degrees with respect to SolarOrbiter and the roll_angle is now taken into account.

• stx_vis_fwdfit_pso: convertion from HPC to STIX coordinate frame (if the (x,y) location is set by the user); upper and lower bound transformation from the Solar Orbiter coordinate frame to the STIX coordinate frame;
• vis_fwdfit_pso and vis_fwdfit_func_pso: roll angle is taken into account. It is also taken into account that the coordinates of the position are searched in a rectangle centered on (0,0).