Improve Yokoya class and use it everywhere needed

[nmounet]

The Yokoya class could get:

• single-plate factors,
• additional tests, e.g. in flat or single plate, dipolar x and quadrupolar x in the plane parallel to the plate(s) should always be exactly opposite to each other,
• elliptic factors.

Also, one should make sure to use it everywhere needed (in e.g. predefined Component objects, in component.py).