Optional arguments for loadtxt

[Beliavsky]

Loadtxt reads an array, which will typically represent data. Often a data file will have have column labels on the first line. I suggest adding an optional nskip_lines argument so that loadtxt can handle such a file by calling it with nskip_lines = 1. One could also add an optional col_labels(:) character array argument, such that
size(col_labels) == size(d,2). Adding these features would allow loadtxt to handle a wider range of data files that people have and give it some of the functionality of read.table() in R. The functions to read data frames in R and Python pandas allow columns to be of different types, which I do not expect loadtxt to handle.

To speed up a program during development one may read only a subset of the data. To accommodate this optional arguments max_rows and max_col could be added. Read.table() in R has an nrows argument, the maximum number of rows to read in.

[MuellerSeb]

See #652 for the skiprows feature.