Implement a GetSize() function for the ImageMetadata class

Summary

Add a GetSize() function to ImageMetadataBase and ImageMetadata.

Rationale

Fix #2237 (closed)

Implementation Details

Classes and files

Update the class ImageMetadataBase that return the count of key:value pairs contained in the metadata maps (NumericKeys, StringKeys, etc).

Update the class ImageMetadata that return the count of key:value pairs contained in all bands and base object.

Tests

Add a test for this functions.

Copyright

The copyright owner is CNES and has signed the ORFEO ToolBox Contributor License Agreement.

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added refactoring label

assigned to @julienosman

requested review from @ctraizet

added Ready for review label

merged

mentioned in commit 7c6de401