uwot 0.2.2

Bug fixes and minor improvements

RSpectra is now a required dependency (again). It was a required dependency up until version 0.1.12, when it became optional (irlba was used in its place). However, problems with interactions of the current version of irlba with an ABI change in the Matrix package means that it's hard for downstream packages and users to build uwot without re-installing Matrix and irlba from source, which may not be an option for some people. Also it was causing a CRAN check error. I have changed some tests, examples and vignettes to use RSpectra explicitly, and to only test irlba code-paths where necessary. See #115 and links therein for more details.