Added a 'discard' argument to Sample._init_() that allows the user to
specify how many samples to discard from the beginning of the generated
Halton sequence (note that this is in addition to the 20*k that are
discarded by default). This makes it possible to continue a sample from
where it was terminated if it's determined that sufficient precision in
the sensitivity calculations hasn't been achieved. Also did some general
code clean up.