stats across regexed field names, string/num stats, CSV UTF BOM strip

This release contains mostly feature requests.

Features:

• The stats1 verb now lets you use regular expressions to specify which field names to compute statistics on, and/or which to group by. Full details are here.

• The min and max DSL functions, and the min/max/percentile aggregators for the stats1 and merge-fields verbs, now support numeric as well as string field values. (For mixed string/numeric fields, numbers compare before strings.) This means in particular that order statistics -- min, max, and non-interpolated percentiles -- as well as mode, antimode, and count are now possible on string-only (or mixed) fields. (Of course, any operations requiring arithmetic on values, such as computing sums, averages, or interpolated percentiles, yield an error on string-valued input.)

• There is a new DSL function mapexcept which returns a copy of the argument with specified key(s), if any, unset. The motivating use-case is to split records to multiple filenames depending on particular field value, which is omitted from the output: mlr --from f.dat put 'tee > "/tmp/data-".$a, mapexcept($*, "a")' Likewise, mapselect returns a copy of the argument with only specified key(s), if any, set. This resolves #137.

• A new -u option for count-distinct allows unlashed counts for multiple field names. For example, with -f a,b and without -u, count-distinct computes counts for distinct pairs of a and b field values. With -f a,b and with -u, it computes counts for distinct a field values and counts for distinct b field values separately.

• If you build from source, you can now do ./configure without first doing autoreconf -fiv. This resolves #131.

• The UTF-8 BOM sequence 0xef 0xbb 0xbf is now automatically ignored from the start of CSV files. (The same is already done for JSON files.) This resolves #138.

• For put and filter with -S, program literals such as the 6 in $x = 6 were being parsed as strings. This is not sensible, since the -S option for put and filter is intended to suppress numeric conversion of record data, not program literals. To get string 6 one may use $x = "6".

Documentation:

• A new cookbook example shows how to compute differences between successive queries, e.g. to find out what changed in time-varying data when you run and rerun a SQL query.

• Another new cookbook example shows how to compute interquartile ranges.

• A third new cookbook example shows how to compute weighted means.

Bugfixes:

• CRLF line-endings were not being correctly autodetected when I/O formats were specified using --c2j et al.

• Integer division by zero was causing a fatal runtime exception, rather than computing inf or nan as in the floating-point case.

Binaries:

As below. Additionally, the MacOSX version is available in Homebrew. For Windows, you need the .exe file along with both .dll files, with instructions as in https://github.com/johnkerl/miller/releases/tag/v5.1.0w.