Exporting the currently loaded network file

This PR adds an ability to export any currently loaded network.
The export_net command now takes an optional filename parameter.
If the loaded net is not the embedded net the filename parameter is required.

Two changes were required to support this:

* the "architecture" string, which is really just a some kind of description in the net, is now saved into netDescription on load and correctly saved on export.
* the AffineTransform scrambles weights for some architectures and sparsifies them, such that retrieving the index is hard. This is solved by having a temporary scrambled<->unscrambled index lookup table when loading the network, and the actual index is saved for each individual weight that makes it to canSaturate16. This increases the size of the canSaturate16 entries by 6 bytes.

closes https://github.com/official-stockfish/Stockfish/pull/3456

No functional change

README.md

@@ -24,13 +24,13 @@ This distribution of Stockfish consists of the following files:
   * Readme.md, the file you are currently reading.
 
   * Copying.txt, a text file containing the GNU General Public License version 3.
-  
+
   * AUTHORS, a text file with the list of authors for the project
 
   * src, a subdirectory containing the full source code, including a Makefile
     that can be used to compile Stockfish on Unix-like systems.
 
-  * a file with the .nnue extension, storing the neural network for the NNUE 
+  * a file with the .nnue extension, storing the neural network for the NNUE
     evaluation. Binary distributions will have this file embedded.
 
 ## The UCI protocol and available options
@@ -156,8 +156,14 @@ For developers the following non-standard commands might be of interest, mainly
   * #### eval
     Return the evaluation of the current position.
 
-  * #### export_net
-    If the binary contains an embedded net, save it in a file (named according to the default value of EvalFile).
+  * #### export_net [filename]
+    Exports the currently loaded network to a file.
+    If the currently loaded network is the embedded network and the filename
+    is not specified then the network is saved to the file matching the name
+    of the embedded network, as defined in evaluate.h.
+    If the currently loaded network is not the embedded network (some net set
+    through the UCI setoption) then the filename parameter is required and the
+    network is saved into that file.
 
   * #### flip
     Flips the side to move.
@@ -189,7 +195,7 @@ Stockfish binary, but the default value of the EvalFile UCI option is the name o
 that is guaranteed to be compatible with that binary.
 
 2) to use the NNUE evaluation, the additional data file with neural network parameters
-needs to be available. Normally, this file is already embedded in the binary or it 
+needs to be available. Normally, this file is already embedded in the binary or it
 can be downloaded. The filename for the default (recommended) net can be found as the default
 value of the `EvalFile` UCI option, with the format `nn-[SHA256 first 12 digits].nnue`
 (for instance, `nn-c157e0a5755b.nnue`). This file can be downloaded from
@@ -202,7 +208,7 @@ replacing `[filename]` as needed.
 
 If the engine is searching a position that is not in the tablebases (e.g.
 a position with 8 pieces), it will access the tablebases during the search.
-If the engine reports a very large score (typically 153.xx), this means 
+If the engine reports a very large score (typically 153.xx), this means
 it has found a winning line into a tablebase position.
 
 If the engine is given a position to search that is in the tablebases, it