Synoptic: Add READ/WRITE PV types to components

In some cases e.g. the frequency of the chopper on the LARMOR synoptic there is a read and a write PV that are closely related. When editing the synoptic these are two separate read/write pvs. This could instead be condensed into a single READ & WRITE PV type. Advantages of this are:
* Brevity in the PV list view for a component
* Direct relation between what is put in the editor and what is in the display (it's unclear to a user that adding a read and write pv of the same name will condense them in the display)
* Currently, if the user makes a typo on one of the read/write pv names they will not get condensed
* Code is more maintainable in displaying synoptic and in making sure there are unique PVs in the synoptic editor.