Add functionality for StatCan trend estimates

StatCan has a standard procedure for trend estimates, @jciconsult shared his R implementation for the code. We could add this to the base package, it is useful for reproducing StatCan results. Or we could start a separate package with helper functions. Thoughts @dshkol?

Implementation below.

#jci implementation of stc trend
stc_trend<-function(x,method=1){
	weights<-c(
	-0.027,	-0.007,	0.031,	0.067,	0.136,	0.188,	0.224,	0.188,	0.136,	0.067,	0.031,	-0.007,	-0.027)
	#first do the base filter
	result1<-rep(NA,length(x))
	if(method %in% c(1,2)){
		result1<-filter(x,weights,sides=2)
		#now do first 6 cells
		for(icell in 1:6){
			cell_weights<-tail(weights,icell+6)
			cell_weights<-cell_weights/sum(cell_weights)
			numb_weights<-length(cell_weights)
			input_data<-head(x,numb_weights)
			cell_value<-sum(cell_weights*input_data)
			result1[icell]<-cell_value
		}
	}
	#now do last 6 cells
	vlength<-length(result1)
	if(method %in% c(1,3)){
		for(icell in 1:6){
		cell_weights<-head(weights,icell+6)
			cell_weights<-cell_weights/sum(cell_weights)
			numb_weights<-length(cell_weights)
			input_data<-tail(x,numb_weights)
			cell_value<-sum(cell_weights*input_data)
			result1[vlength-icell+1]<-cell_value
		}
	}
	stc_trend<-result1
}

Start by reviewing the proposed R implementation in the issue and the existing cansim package structure. Resolve whether the functionality belongs in the base package or a separate helper package, then define how its results will be validated against StatCan trend estimates. The issue names no files or tests, so an entry point and completion criteria still need to be established.