compute: move numeric conversion into 'field-ops' module.
fields-ops: remove 'keep_lines' feature.

Perhaps will be added in future versions.
compute: detect invalid suffix in numeric input.
build: Use compiler warnings from gl_WARN_ADD

Those were wrongly dropped when changed to non-recursive makefile.

The new flags are used only with the 'compute' sources,
but not with the gnulib sources.
build: improve comment in configure.ac
compute: set numeric output precision
compute: new operations: first,last
tests: fix comments
build: add -Werror to ./src/ files.
compute: new operation: rand
compute: new operations: q1,q3,iqr
tests: add tests for pop/sample stdev/variance
compute: new operations: mad,madraw
tests: document equivalent R code for stat tests + reorder.
src/utils: extract function for arithmetic mean.
compute: new operation: skewness (for pop./sample)
compute: new operation: excess kurtosis
tests: tests more sequences
tests: test mad/madraw with unsorted input
compute: new operation: Jarque-Bera normality test