MCS modification functions

[lrennels]

• write some documentation for the MCS modification functions

• make sure all MCS modification functions have proper errors if a RV is not found ex. add an error if :replace_RV is called for a parameter that does not exist, right now it adds one which is confusing and unexpected behavior

• write a test file for MCS modification functions

• explicitly note that if you don't create names for your random variables automatic ones are created and maybe nondeterministic ... good idea to at some point add a function to do something like ...

mcs = getsim()
replace_RV!(mcs, :name, Normal(1,2)

when :name was defined with the shorthand

name = Normal(0,1)

and thus has a automatically generated key in mcs.rvdict in the format :name!number