Allow output without time specifier

Related to #27

Now one can set two types of writers. Writers with a specified output
time period (the was the previous behavior) and writers without time
info.

The output filenames are simpler in the second case since they do not
include the output step number in their names.

To these writers are now associated two `write` functions. The first
one (legacy one) specifies the time of the output. The second one (new
one) does not expect time value.