Simplify syntax to get data from PTDATA

[yumouwei]

This PR allows a user to use

conn.get_data('ip', tree_name='ptdata')

to replace

conn.get_data(f'ptdata("ip", {params.shot_id}, tree_name='d3d')
conn.get_data(f'ptdata("ip", {params.shot_id}, tree_name=None)

For the moment, this is only needed for using SignalTimeSetting with PTDATA signals on DIII-D. Therefore I did not modify any expression in d3d/physics.py or anywhere else. It also only works with simple expressions (i.e. it won't work with additional arithmetic operations).

[yumouwei]

I just found out that conn.get_data(f'ptdata("ip", {params.shot_id}, ...') doesn't need specifying tree_name. We should remove all the tree_name="d3d" to save the time used in opening those trees.

[gtrevisan]

the inout module is machine-agnostic, could we move this somewhere else in the DIII-D-specific module? maybe a helper function somewhere?

[yumouwei]

In terms of using PTDATA signal in SignalTimeSetting, for now this is addressed by the _get_times_d3d_wrapper added in #439. We decided not to modify mds.py following the above discussion.