runtime: added ability for blocks to have their own max_noutput_items.

Setting a max_noutput_items for an individual block will override the global max that can be passed to the start/run method of the top_block.

Also adds QA code for testing that these features run and complete a flowgraph.
docs: added a description of a basic flowgraph to the manual's main page.

Also a brief discussion of throughput/latency and how to use the max_noutput_items methods to control them.
core: exported max_noutput_item methods from gr_block to Python.
docs: added section explaining the basics of reconfiguring a flowgraph.
blocks: adds new top-level component gr-blocks with first block
	gr::blocks::add_XX
blocks: added blocks_add_const_xx
blocks: fix XML file in previous commit
blocks: copy alignment fix from maint
Merge commit 'v3.6.1' into wip/gr-blocks-master
blocks: added multiply_xx, some cleanup on add_xx
blocks: completed transition of add and multiply blocks

* gr::blocks::add_*
* gr::blocks::add_const_*
* gr::blocks::add_const_v*
* gr::blocks::multiply_*
* gr::blocks::multiply_const_*
* gr::blocks::multiply_const_v*

Each of these has ss, ii, ff, and cc versions, for a total of
24 blocks.
blocks: added div_XX and sub_XX blocks
blocks: added gr::blocks::char_to_float
Merge branch 'master' into wip/gr-blocks-master
Merge branch 'master' into wip/gr-blocks-master
blocks: added gr::blocks:char_to_short
Merge branch 'master' into wip/gr-blocks-master
blocks: added gr::blocks::complex_to_interleaved_short
blocks: added gr::blocks::complex_to_float
blocks: added gr::blocks::complex_to_real