optimization method options should be better documented

[willwerscheid]

even a basic description of what the options mean would be helpful; at present there is nothing

[pcarbo]

@willwerscheid I pushed a couple commits. How's this?

optmethod: The method used to compute maximum-likelihood estimates of the mixture weights. The default setting,
“mixSQP”, uses the fast sequential quadratric programming (SQP) method implemented in the mixsqp package.
Alternative methods include the interior-point method implemented in the REBayes package (optmethod = "mixIP"),
and a simple Expectation Maximization (EM) algorithm (optmethod = "mixEM"). For more details on the different
options, see the help for functions estimate_mixprop, mixSQP, mixIP, mixEM, w_mixEM and mixVBEM.

[pcarbo]

@stephens999 Your review on this change is welcome, but not necessary.

[willwerscheid]

@pcarbo Looks great! Those functions mentioned at the end are all linked, correct?

[pcarbo]

They should be, yes. You can reinstall the package if you want to check this.