next_block: Read the next block of data from a file.
In LaF: Fast Access to Large ASCII Files

Read the next block of data from a file.

next_block(x, ...)

## S4 method for signature 'laf'
next_block(x, columns = 1:ncol(x), nrows = 5000, ...)

## S4 method for signature 'laf_column'
next_block(x, nrows = 5000, ...)

`x`	an object the supports the `next_block` method, such as an `laf` object.
`...`	passed to other methods. Reads the next block of lines from a file. The method returns a `data.frame`. The first line in the `data.frame` is the line corresponding to the current line in the file. When the end of the file is reached a `data.frame` with zero rows is returned. This can be used to check whether the end of the file is reached.
`columns`	an integer vector with the columns that should be read in.
`nrows`	the (maximum) number of rows to read in one block