fixed R CMD build issues
'tabs' are now 2 spaces
some tests have been removed because 1) random systems were causing issues due to reaching the maximum number of unsuccessful tau_leap attempts and 2) an sckm system no longer carries lmult but rather just mult and lower level functions would have to be rewritten to calculate lmult if it isn't already there and this seems unnecessary
default engine changed to C now that R and C match
default engine changed in documentation too
added SEIR model