Merge pull request #11 from cloudygoose/master

add matrix doc

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+#The Nerv Matrix Package#
+Part of the [Nerv](../README.md) toolkit.
+
+##Description##
+###Underlying structure###
+In the begining is could be useful to know something about the underlying structure of a __Nerv__ matrix.  
+Every matrix object is a encapsulation of a C struct that describes the attributes of this matrix.  
+```
+typedef struct Matrix {
+    size_t stride;              /* size of a row */
+    long ncol, nrow, nmax;    /* dimension of the matrix */
+    union {
+        float *f;
+        double *d;
+        long *i;
+    } data;                   /* pointer to actual storage */
+    long *data_ref;
+} Matrix;
+```
+It is worth mentioning that that `data_ref` is a counter which counts the number of references to its memory space, mind that it will also be increased when a row of the matrix is referenced(`col = m[2]`). A __Nerv__ matrix will deallocate its space when this counter is decreased to zero.   
+Also note that all assigning operation in __Nerv__ is reference copy, you can use `copy_tod` or `copy_toh` method to copy value.
+
+###Class hierarchy###
+The class hierarchy of the matrix classes can be clearly observed in `matrix/init.c`.   
+First there is a abstract base class __Nerv.Matrix__, which is inherited by __Nerv.CuMatrix__ and __Nerv.MMatrix__(also abstract).  
+Finally, there is __Nerv.CuMatrixFloat__, __Nerv.CuMatrixDouble__, inheriting __Nerv.CuMatrix__, and __Nerv.MMatrixFloat__, __Nerv.MMatrixDouble__, inheriting __Nerv.MMatrix__.
+
+##Methods##
+Mind that usually a matrix object can only do calculation with matrix of its own type(a __Nerv.CuMatrixFloat__ matrix can only do add operation with a __Nerv.CuMatrixFloat__).  
+In the methods description below, __Matrix__ could be __Nerv.CuMatrixFloat__, __Nerv.CuMatrixDouble__, __Nerv.MMatrixFloat__ or __Nerv.MMatrixDouble__.   
+* __Matrix = Matrix(int nrow, int ncol)__  
+Returns a __Matrix__ object of `nrow` rows and `ncol` columns.
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