Add bi-annual frequency.

See merge request Dynare/dseries!37

It was no more possible to fill a variable with NaNs, after the switch
to the new behaviour of the merge method (see test #5 in
@dseries/merge.m). Added a legacy mode to the merge method to
circumvent the issue (@dseries/subsasgn sets legacy equal to true when
calling the merge method).

Second argument of merge overwrites its first argument except if
second argument has NaNs (in this case first argument takes
precedence). This commit reverts fd7476e6 and adds a new unit test.

Normally it should still be possible to instantiate dseries objects
from mat files generated by previous versions of Dynare (before 4.6).

warning doesn't print sprintf characters unless it is given more than one argument. hence use sprintf

If a variable, say x, is defined both in dseries objects o and p, then
merge(o, p) will select the variable x as defined in the second input, p,
except for NaNs elements in p if corresponding elements in the first
input, o, are numbers.

— On GNU/Linux, the locally installed binary now has priority over the
  system-wide binary
— Do not require 32-bit binary if we are on a 64-bit system and the 64-bit
  binary is there (or vice versa)
— Code factorization in the detection logic

Closes: #41

On Windows and macOS, those binaries are now bundled in packages. On Debian,
one can use the official x13as package.