Fix #206 Determine --maxSharedMemory automatically, if not set by user.

[vasdommes]

If --maxSharedMemory=0 (set by default), SDPB will try to determine it automatically.

SDPB will estimate how much memory is required to allocate all matrices used by solver, and set the limit for shared windows (used in bigint_syrk_blas() for calculating Q matrix) to 50% of the remaining memory.

Note that the memory estimates are rather imprecise, and sometimes one has to set a lower limit (e.g. 25%) to prevent OOM.
Choosing 50% for automatic limit is somewhat arbitrary. User should use this value as a starting point and decrease it when necessary. I added these notes to Usade.md.

Note also that the limit is set for each node separately, because each node has its own shared windows and may have blocks of different size.

Example output:

Warning: node=0: 
	MemTotal: 251.5 GB
	Required memory estimate (excluding shared memory windows): 81.5 GB
	To prevent OOM, SDPB will set --maxSharedMemory to 50% of the remaining memory, i.e. 85.0 GB
	In case of OOM, consider increasing number of nodes and/or decreasing --maxSharedMemory limit.

In debug mode, SDPB will also print matrix sizes, e.g.

node=4 matrix sizes and memory estimates: 
	#(SDP) = 134307093
	#(X) = 2347739
	#(A_X_inv) = 9390902
	#(schur_complement) = 28771896
	#(B) = 128743304
	#(Q) = 3723776
	BigFloat size: 168 B
	Total BigFloats to be allocated: 359533942 elements = 56.3 GB
	Initial MemUsed (at SDPB start) = 25.9 GB
	Total non-shared memory estimate: 82.1 GB (88187737456 bytes)

and matrix allocation messages, e.g.

node=0: allocate schur_complement: 9.1 GB
node=0: allocate schur_off_diagonal: 35.3 GB