[ADD] Nested (MATNEST-style) block matrix support and tests

Add a block-structured distributed operator that presents itself to Krylov
solvers and preconditioners as a single ordinary distributed matrix (the
PSBLAS analogue of PETSc MATNEST), targeting saddle-point systems
M = [[A, B^T], [B, 0]] with possibly rectangular sub-blocks.

Library (base/modules):
- psb_desc_nest_mod, psb_d_nest_mat_mod: grid of per-field descriptors and
  per-block sparse storage.
- psb_d_nest_base_mat_mod: psb_d_nest_base_mat, the operator extending
  psb_d_base_sparse_mat (local csmv, free, field-split hooks for a future
  block preconditioner).
- psb_cd_nest_tools_mod / psb_d_nest_tools_mod: composed global descriptor
  with union halo (psb_cd_nest_compose) and rectangular local block builder
  (psb_d_nest_rect_block), plus the per-block assembly wrappers.
- psb_d_nest_builder_mod: psb_d_nest_matrix, the user frontend with the
  init/ins/asb/free pattern hiding all descriptor/halo/compose/setup
  boilerplate.
- psb_d_nest_mod: umbrella module (use psb_d_nest_mod).

Remove the earlier bespoke per-block prototype (comm/psblas/vect modules and
the pde_nest_psblas test) superseded by the single MATNEST design.

Tests (test/nested): glob (square operator vs monolithic CSR oracle), rect
(genuinely rectangular blocks), cg (low-level path, ill-conditioned SPD
red-black Laplacian solved with standard CG), builder (same solve via the
utility), plus a README describing the design and usage. All pass serially
and in parallel, with results invariant to the process count.

Build hooks updated (autotools Makefiles + CMakeLists); the nested tests are
relocated out of test/pdegen into test/nested.

Author: Simone Staccone (Stack-1)

test/nested/Makefile

@@ -0,0 +1,46 @@
+INSTALLDIR=../..
+INCDIR=$(INSTALLDIR)/include
+MODDIR=$(INSTALLDIR)/modules/
+include $(INCDIR)/Make.inc.psblas
+#
+# Libraries used
+LIBDIR=$(INSTALLDIR)/lib
+PSBLAS_LIB= -L$(LIBDIR) -lpsb_util -lpsb_linsolve -lpsb_prec -lpsb_base
+LDLIBS= $(PSBLDLIBS)
+#
+# Compilers and such
+#
+CCOPT= -g
+FINCLUDES=$(FMFLAG)$(MODDIR) $(FMFLAG).
+
+
+EXEDIR=./runs
+
+all: runsd psb_d_nest_glob_test psb_d_nest_rect_test psb_d_nest_cg_test psb_d_nest_builder_test
+
+runsd:
+	(if test ! -d runs ; then mkdir runs; fi)
+
+psb_d_nest_glob_test: psb_d_nest_glob_test.o
+	$(FLINK) psb_d_nest_glob_test.o -o psb_d_nest_glob_test $(PSBLAS_LIB) $(LDLIBS)
+	/bin/mv psb_d_nest_glob_test $(EXEDIR)
+
+psb_d_nest_rect_test: psb_d_nest_rect_test.o
+	$(FLINK) psb_d_nest_rect_test.o -o psb_d_nest_rect_test $(PSBLAS_LIB) $(LDLIBS)
+	/bin/mv psb_d_nest_rect_test $(EXEDIR)
+
+psb_d_nest_cg_test: psb_d_nest_cg_test.o
+	$(FLINK) psb_d_nest_cg_test.o -o psb_d_nest_cg_test $(PSBLAS_LIB) $(LDLIBS)
+	/bin/mv psb_d_nest_cg_test $(EXEDIR)
+
+psb_d_nest_builder_test: psb_d_nest_builder_test.o
+	$(FLINK) psb_d_nest_builder_test.o -o psb_d_nest_builder_test $(PSBLAS_LIB) $(LDLIBS)
+	/bin/mv psb_d_nest_builder_test $(EXEDIR)
+
+clean:
+	/bin/rm -f psb_d_nest_glob_test.o psb_d_nest_rect_test.o psb_d_nest_cg_test.o psb_d_nest_builder_test.o *$(.mod) \
+	$(EXEDIR)/psb_d_nest_glob_test $(EXEDIR)/psb_d_nest_rect_test $(EXEDIR)/psb_d_nest_cg_test $(EXEDIR)/psb_d_nest_builder_test
+verycleanlib:
+	(cd ../..; make veryclean)
+lib:
+	(cd ../../; make library)