removing fixed variables/NLP preprocessing [on device]

[cnpetra]

parts of the code NLP preprocessing code need to be revisited since they are "wired" to work only on CPU. Need to either use generic hiopVector kernels or port these parts using Raja. This is especially pervasive in fixed variables removal.

@pelesh

[pelesh]

@cnpetra, in #153 I set in hiopOptions::ensureConsistence() to replace "remove" with "relax" option for fixed variables whenever GPU mode is enabled. This may be sufficient to address this issue at least for the time being. Removing fixed variables on GPU device would require more refactoring.

[cnpetra]

closing this since it is partially solved and not of relevance for device computations.