Update NTScalar display to include precision and form

[thomascobb]

The (definitive, prospective, speculative?) NT wiki conversion https://github.com/epics-base/normativeTypesCPP/wiki/Normative+Types+Specification#display_t shows a display_t as having a precision and form rather than a format string.

p4p has the old format string:
https://github.com/mdavidsaver/p4p/blob/master/src/p4p/nt/scalar.py#L123

Is precision and form the new standard? If so, can we update? Shall I have a go at it?

[mdavidsaver]

Is precision and form the new standard?

Yes. This is the intent anyway.

If so, can we update?

Attempts to update the NT* definitions have run into the problem of breakage with clients using the normativeTypesCPP module with its extremely strict (and often unnecessary) type checks epics-base/normativeTypesCPP#15. What was intended as a linter check ended up being applied by regular clients.

Since epics-base/pvDataCPP#62 it is probably reasonable to update P4P as well.

Shall I have a go at it?

Up to you.