UMAP: Remove mutable

[weefuzzy]

UMAP::mEmbedding had been marked mutable in order to avoid some code duplication. mutable shouldn't be used in the absence of an accompanying lock if the principle of least astonishment is to be honoured in a multi-threaded situation.

The problem for the original author was that a long and complex function optimizeLayout needs to be called both in mutating and non-mutating circumstances, and buried within nested loops is an optional update of this variable. In order for the function to be callable in a const case, it needed to be marked const, but also needed to be able to update mEmbedding during training. It's certainly an amount / complexity of code that one doesn't want to duplicate.

Without more involved refactoring, I think the best we can do here is to:

• hoist the function out of the class as a function template taking the reference embedding as a template parameter (so it can be const or not).
• use a compile time check on the const-ness of the reference embedding to enable / disable the option to update.
• provide more clearly labelled and properly const (or not) member functions that delegate to the full implementation (optimizeLayout and optimizeLayoutAndUpdate).

fixes #224

[tremblap]

applied over PR #227 and it works on all 3 OSes on SC