Refactor UKF for library usage

[ihnorton]

Refactoring steps for usage from both CLI (executable) and a separate shared library module interface.

[ihnorton]

cc @ljod @yrathi these are the core changes we needed for the new module work. All tests pass so I will go ahead and merge, but I set up a separate Slicer 4.8 release branch to build from the prior commit, so anyone who needs more stability can use that. I may send a message on the forum to recommend using that for the next few weeks.

• maintenance branch: https://github.com/pnlbwh/ukftractography/tree/release-4.8
• index commit: https://github.com/Slicer/ExtensionsIndex/commit/c19728572d333b2c954267a5f7c87ab5f788a39d
• release-4.8 build was successful on that branch last night.