Add handling of ItemValue and SubItemValuePerItem into the language

See merge request !148

- SubItemValuePerItem is associated to the pugs abstract type sub_item_value
- ItemValue is associated to the pugs abstract type item_value

This is done to enrich the data flow between functions but cannot
really be manipulated in the language.

However, one can interpolate to ItemData and can post process
them (only CellValue and NodeValue are supported by VTK and Gnuplot
writers).

- doc and tests have been updated

Oversubscription can degrade parallel performances substantially

Take into account suggestions of Emmanuel

See merge request !147

This includes an important change in the code since one cannot
reuse a variable name in a nested block (this means that if a variable
is declared, one cannot declare a variable with a same name even in a
nested block, while the first variable exists).

Refactor slightly BuiltinFunctionEmbedder and check types

Closes #29

See merge request !146

Now the arguments and return values types are checked at
construction. One checks if the C++ type corresponds to a pugs type.

Closes #29

git subrepo clone git@gitlab.com:OlMon/org-themes.git packages/org-themes

Closes #27 and #26

See merge request !145

This duplication can occur when intercepting a NormalError

Remove legacy "in code" output.
It is still possible to create output files directly from C++, but
quantities have to be added one by one, not at construction anymore.

This finally closes #27

Also add a test to check that used names are unique

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.

Related to #27

These functions were created while builtin functions polymorphism was
not implemented

For instance, tuples of Z where denoted (Z...) by the mangler, it is
now defined following the language syntax as (Z)

core and math modules have been described