Prepare for deprecation of the blocks argument

This PR prepares for the deprecation of the blocks argument at various places.

Removes the need for blocks in initialize_chain()
In rbind(), when formulas are concatenated and duplicate names are found, also rename the duplicated variables in formulas by their new name
Fixes a bug in filter.mids() that incorrectly removed empty components in the imp object
Fixes a bug in ibind() that incorrectly used length(blocks) as the first dimension of the chainMean and chainVar objects
Corrects the description visitSequence, chainMean and chainVar components of the mids object

amices / mice