Pythonic radDataPtr

Functions in radDataRead are now thin wrappers over methods associated
with radDataPtr class.

Noteworthy changes:

* radDataPtr dmapio now supports seeking to specific dmap record using
 an index lookup. And rewinding back to the beginning of the ptr object.

 radDataPtr can now build index of dmap record offset using record
 datetime as lookup key. Similar scanboundaries can be indexed.
 Indexing is prereq functionality for interactive data viewer which will
 allow you, among other things, to point and click on pixels in an fit
 variable RTI canvas and seek directly to the corresponding rawacf record
 if available.

* python file pointers have been replaced with OS file descriptors to
 make it possible for C code in dmapio and python code to correctly
 seek/tell inside an open file. python file pointers, being abstracted
 python objects don't seek/tell like C FILE pointers. Passing file
 descriptors into the C code as the underlying representation solves the problem.

* Some attributes of radDataPtr have been converted to private
 attributes to discourage users from interacting with them directly.
 For example the fd, fp and filename are considered private and are
 manipulated only via class methods and should not be manipulated
 directly. As manipulating directly will result in unexpected behavior
 if fd,fp and filename go out of sync.

* functions in radDataRead should continue to work as expected for
 backwards compatiblility for any code that does not want to take
 advantage of the radDataptr methods directly.

 On branch Feature_Python_RadarDataPtr
 Changes to be committed:
	modified:   pydarn/rst/src/convert.c
	modified:   pydarn/rst/src/dmap.c
	modified:   pydarn/rst/src/dmap.h
	modified:   pydarn/rst/src/dmapio.c
	modified:   pydarn/rst/src/rtypes.h
	modified:   pydarn/sdio/radDataRead.py
	modified:   pydarn/sdio/radDataTypes.py
	modified:   pydarn/sdio/sdDataRead.py
	modified:   pydarn/sdio/sdDataTypes.py