conversion code should be refactored

[Dimitar Misev]

In many cases, existing code in source:conversion can be removed and the corresponding function can be aliased to encode(). With this we can also remove a couple of direct dependencies of rasdaman as they become indirect dependencies via gdal: bmp, jpeg, png, tiff.

These functions can be refactored:

• bmp
• jpeg
• png
• tiff

Probably not:

• hdf
• netcdf

Not sure about:

• dem
• ecw
• ntf
• tor (can't find information on a tor format)
• vff (can't find information on a vff format)

Not compiled:

• nitf.cc

Not compiled, and I'm not sure what their use would be in rasdaman:

• des.cc
• graphic.cc
• image.cc
• int16.cc
• res.cc
• utilities.cc

Furthermore, it would be good to remove the dedicated source:insertutils/insertppm.cc and either have a corresponding ppm/inv_ppm functions or connect to decode/encode(..,"ppm")

See also #1139