PDK variables to be modified in the flow by OpenLane users

[mo-hosni]

Description

Most of the PDK variables for sky130 or gf180mcu should not be changed by the users in flow, while some variables are very useful to be modified in the flow.

Proposal

Divide the PDK variables into 2 groups, one that the users are encouraged to modify when needed, and the other is specific to the pdk.
This is a list of the pdk variables that can be modified by the users:

FP_PDN_HSPACING
FP_PDN_VSPACING
FP_PDN_HOFFSET
FP_PDN_VOFFSET
FP_PDN_HPITCH
FP_PDN_VPITCH
FP_PDN_HWIDTH
FP_PDN_VWIDTH
FP_PDN_CORE_RING_VWIDTH
FP_PDN_CORE_RING_HWIDTH
FP_PDN_CORE_RING_VSPACING
FP_PDN_CORE_RING_VSPACING
FP_PDN_CORE_RING_VOFFSET
FP_PDN_CORE_RING_HOFFSET
CTS_MAX_CAP
GRT_LAYER_ADJUSTMENTS
RT_MIN_LAYER
RT_MAX_LAYER
FP_IO_VLAYER
FP_IO_HLAYER
FP_TAPCELL_DIST
GPL_CELL_PADDING
DPL_CELL_PADDING
NO_SYNTH_CELL_LIST
DRC_EXCLUDE_CELL_LIST

Also, SYNTH_MAX_TRAN on top of the list after it is moved to the pdk variables.