refactor MFX carriers

[rickwierenga]

https://discuss.pylabrobot.org/t/mfx-carrier/37

[rickwierenga]

most of the diff is just renaming things

[rickwierenga]

• expanded ResourceHolder (formerly ResourceHolderMixin, see Machine no longer a Resource, ResourceHolder is a Resource #281) to include a property for the child resource
• removed CarrierSite, replaced with ResourceHolder
• Carriers now store the sites in a dict instead of a list
• renamed PlateCarrierSite to PlateHolder
• ResourceStack and Plate are not longer ResourceHolders, they just use get_child_location directly
• rename create_homogeneous_carrier_sites to create_homogeneous_resources, parameters site_size_{x,y} to resource_size_{x,y}
• rename create_carrier_sites to create_resources
• directly initialize MFXCarrier with sites rather than having CarrierSite in between

[BioCam]

Going through the changes I couldn't see how the replacement of PlateCarrierSite to PlateHolder maintains the pedestal_z_height logic from PlateCarrierSite?

[rickwierenga]

PlateCarrierSite was simply renamed into PlateHolder, the implementation should be equivalent