started functionality for write to file

as of right now -- only one-file-per drop output style. no data
descriptions in first column of .csv because of issues with strings and
cell-matrix and dlmwrite(). also, don't have checks to see if there is
triaxial load data & just assumes that it's there -- that will break if we
ever don't have it -- probably just needs an if statement and a different
write statement if there isn't triaxial load.

also got rid of dropSet constructor comment because it was out of date and
just confusing at this point.
technically have two write methods

they're clunky and gross and i should probably be ashamed of them.

i did put in checks for triax load though -- not tested.

additionally -- we may be moving towards a need for a 'data' helper class
for drop with methods like data_matrix() -- so that the write methods are
more modular. right now if someone added a new calculated data they would
also have to modify the write statements in drop and that's just bad
design.