Add 'gpu' to 'compute_mode' options

[pelesh]

This pull request adds a GPU compute mode as a HiOp user option. The GPU compute mode runs HiOp kernels on GPU using RAJA library and uses GPU-enabled linear solver. In addition to that, this PR provides some minor fixes:

• Method finalizeInitialization() in hiopNlpFormulation class sets memory space correctly.
• Cleaned up RAJA version of Example 4 (user should not touch linear algebra factory)
• Fixed Inability to select pinned memory space #102
• Replaces "remove" with "relaxed" option for fixed variables when GPU mode is enabled (see also removing fixed variables/NLP preprocessing [on device] #94)
• Updated CMake finder for COINHSL to deal with myriad of different paths where COINOR packages HSL library.

This needs to be merged before we can address #152.

[pelesh]

CC @nychiang