[DOC] Initial guidelines on porting Numpy to DaphneLib.

- Added a page in the user documentation on manually porting Numpy-based Python scripts to DaphneLib-based Python scripts.
- By no means complete, just an initial write-up.
- Added links to the new page in the documentation.
- Added a new method on matrices in DaphneLib: asType() for casting to another data/value type.

doc/DaphneLib/APIRef.md

@@ -149,6 +149,7 @@ In the following, we describe only the latter.
 - **`order`**`(colIdxs: List[int], ascs: List[bool], returnIndexes: bool)`
 
 **Data preprocessing:**
+
 - **`oneHot`**`(info:matrix)`
 - **`bin`**`(numBins:int, Min = None, Max = None)`
 
@@ -162,6 +163,10 @@ In the following, we describe only the latter.
 - **`print`**`()`
 - **`write`**`(file: str)`
 
+**Conversions and casts:**
+
+- **`asType`**`(dtype=None, vtype=None) -> Matrix`
+
 ### `Frame` API Reference
 
 **Frame dimensions:**