restructure double_tank, mv parts to tank_functions

initialize a HybSysDyn in double tank,
pass it to tank_functions,
which now are modified towards processing any number of modes.

the moved code is general, to be used with any HybSysDyn.
label the merged partition with APs
new: AbstractSysDyn.__str__
add title to PropPreservingPartition.__str__
rename variable output -> s
shorten printed horizontal lines