fread empty files

[brry]

readDWD has the argument fread to read datasets through data.table::fread, which is significantly faster than base unzip+read.table.
Early on (2017), the default was fread=NA (i.e. conditional on availability of data.table).
Some users sent emails about errors on Windows OS so I changed the default to FALSE (272b947).
For speedup, it would be nice to have the default NA again.

This will be set experimentally to see if new complaints arise.

[brry]

Here's the error structure reported by at least 2 people independently:

Error in data.table::fread(paste("unzip -p", f, fp), na.strings = na9(), : File is empty: [Rtempfile]
In addition: Warning messages:
1: running command 'C:\Windows\system32\cmd.exe /c (unzip -p [path].zip [pfile].txt) > [Rtempfile] had status 1
2: In shell(paste("(", input, ") > ", tt, sep = "")) : '(unzip -p [path].zip [pfile].txt) > [Rtempfile]' execution failed with error code 1

[pfile] being the produkt file, e.g. produkt_klima_tag_19370101_19860630_00001.txt
for [path] DWDdata/daily_kl_historical_tageswerte_KL_00001_19370101_19860630_hist.zip
or produkt_klima_tag_20160225_20170827_03987.txt for DWDdata/daily_kl_recent_tageswerte_KL_03987_akt.zip

[brry]

For 10 minute data, there is 64-bit formatted columns needing the package bit64 if fread=TRUE/NA.
See 5a3f38e

[brry]

A new error message was reported in #23, but I think it's the same problem.

'unzip' is not recognized as an internal or external command, operable program or batch file.
'(unzip -p [path].zip [pfile].txt) > [Rtempfile]' execution failed with error code 1
File '[Rtempfile]' has size 0. Returning a NULL data.frame. File contains no rows: [path].zip

It seems the system command unzip is not available by default on Windows (source).
On my computer, it is found in Rtools:
Sys.which("unzip") gives me c:\\Rtools\\bin\\unzip.exe