change prefix behaviour

[sjanssen2]

This is addressing #8
My problem was that I misunderstood the meaning of the prefix parameter. Documentation says prefix for the output figure. Coming from gcc compilations, I gave an absolute path as the prefix. But I figure it is intended to be "just" a name.
Hence, the double concatenation of prefix in e.g.

MetaLonDA/R/Metalonda.R

Line 336 in 2ad9428

write.csv(summary.tmp, file = sprintf("%s/%s_MetaLonDA_TimeIntervals.csv", prefix, prefix), row.names = FALSE)

results in an impossible or at least non existent file path.

With this #PR, I'd like to suggest to slightly change the meaning of prefix into a file path.