Interconnect model files unusable in v3.3.0!

[capn-freako]

(Posting for Dennis Han.)

With the release of v3.3.0 we lost the ability to use external interconnect models.
We can still load the files, but the Use file checkbox remains grayed out and un-checkable.

[capn-freako]

I've confirmed Dennis' observation and am working on a fix.

[capn-freako]

Dennis Han has reported that all the relevant "knobs" are inactive (i.e. - grayed out), when attempting to use an interconnect description file.
I've confirmed this.
It looks like I recently introduced a chnl_valid trait (probably wanting to provide some consistency between interconnect and IBIS(-AMI) file handling), but never implemented the logic to support this new trait:

dbanas@Davids-MBP:~/Documents/Projects/PyBERT
$ git log -Schnl_valid pybert/pybert.py
commit e1b93aebdcb60f2fe0bae4f3e838f890d0ecb957 (tag: v3.3.0rc1)
Author: David Banas <david@luminouscomputing.com>
Date:   Sun Nov 10 16:08:23 2019 -0800

    First release candidate for v3.3.0.

dbanas@Davids-MBP:~/Documents/Projects/PyBERT
$ git diff e1b93ae~ e1b93ae pybert/pybert.py | grep -C 5 'chnl_valid'

     # - Channel Control
+    ch_file = File(
+        "", entries=5, filter=["*.s4p", "*.S4P", "*.csv", "*.CSV", "*.txt", "*.TXT", "*.*"]
+    )                          #: Channel file name.
+    chnl_valid = Bool(False)   #: Channel file is valid.
     use_ch_file = Bool(False)  #: Import channel description from file? (Default = False)
     Zref = Float(100)          #: Reference (or, nominal) channel impedance.
     padded = Bool(False)       #: Zero pad imported Touchstone data? (Default = False)
     windowed = Bool(False)     #: Apply windowing to the Touchstone data? (Default = False)
     f_step = Float(10)         #: Frequency step to use when constructing H(f). (Default = 10 MHz)dbanas@Davids-MBP:~/Documents/Projects/PyBERT

Note: The ch_file trait was not newly added, just moved.

I think the easiest/quickest fix is to just remove the new chnl_valid trait and all dependencies on it (i.e. - revert to the old behavior)...
Yep, that seems to have worked; releasing a fix as: v3.3.1.