Added a taller microscope stand

If you use the infinity corrected optics module (optics_picamera2_rms_f50d13_infinity.stl), the camera hits the Raspberry Pi. This adds a 45mm tall version of the microscope stand, to give an extra 15mm of space for the longer tube.

@kasbah this would require an update to the STL selector, once it's merged - the 45mm tall microscope_stand is the right microscope_stand when using the _infinity optics modules, the 30mm tall one is fine for other versions (though someone reported camera clash with that too, so it might be worth raising it up another few mm).