Instance parameters for XSPICE code models
==========================================

Xspice codemodels can now have instance parameters.
Codemodels instances can now also use a default model.
Two new codemodels are added: da_switch and d_const

Usage from netlist:
-------------------
a<instname> <connections> <model> <instparam>=<value(s)>
If <model> starts with !, the corresponding codemodel device name will be found
and a default model will be automatically created. Otherwise <model> must be a model name declared by a .model card.
Example:
asum [n1 n2 n3] vsum summermod in_gain=1,1,2 out_gain=2 
.model summermod summer
alut [d1 d2 d3] qlut !d_lut table_values="01101001"

Writing code models with instance parameters:
---------------------------------------------
Parameters that can be set on instances must be declared in the ifspec.ifs file
in the corresponding PARAMETER_TABLE under the new Parameter_Scope keyword.
The value can be "model", "instance", or "both".
 model    : parameter can be set only to a model
 instance : the parameter can be set to instances, but not on models
 both     : the parameter can be set either to instances or models, or both.
Default scope is "model".
If the scope is "both" and the instance parameter is not set, the value of the
corresponding model parameter will be used. If the parameter is not set on the model either, the default value will be used.

This is nothing special to do in the cfunc.mod file, the same PARAM() access will works for all parameters. The codemodel however need to be recompiled.
(you need to touch cfunc.mod to trigger a re-compilation even if ifspec.ifs is modified).

Binary compatibility of codemodels
----------------------------------
Codemodels compiled for older version of ngspice should be compatible with the
new version.
Codemodels compiled for the new version of ngspice and which use
"instance" or "both" scope for any parameter will not be compatible with older version of ngspice that do not have this feature.

If you have several version of ngspice installed, beware of which codemodels
are loaded by the spinit file. It might be useful to specify explicitely a specific spinit file which points to the codemodel you want to load.

The default parameter scope is "model" if not specified in ifspec.ifs, this
ensure backward and forward compatibility for unmodified code models.

The following codemodel was modified for having parameters with "both" scope:
analog.cm: summer, slew, mult, int, gain, climit
digital.cm: d_dff, d_dlatch, d_jkff, d_srff, d_srlatch, d_tff (ic parameter)
digital.cm: d_genlut, d_lut, d_source

New codemodels
--------------
da_switch: analog switch controlled by digital node
d_const: constant digital source with vector output