RSF examples fix (#306)

* Added RF writer and tests

* fixed RSF writer to write esize=4

* made all outputs to RSF as native_float. Updated comments and notes.

* added test and ValueError for complex data. RSF writer only writes int or float data now.

* RSF writer now allows to output in or float. Clause added to fail if not those two types.

* fix Examples in documentation

dascore/io/rsf/__init__.py

@@ -12,8 +12,9 @@
 
 # get the path to a random DAS file.
 patch = dc.get_example_patch()
+spool = dc.spool(patch)
+spool.io.write("test_out.rsf","rsf",data_path="test_out.rsf")
 
-patch.io.write("test_out.rsf","rsf",data_path="test_out.rsf")
 """
 
 from .core import RSFV1