libCOMBINE: allow a COMBINE archive to be initialised using some file contents

[agarny]

Right now, the only way to initialise a COMBINE archive using libCOMBINE is by providing a file name. However, this means that we can't handle COMBINE archives from JavaScript, unlike for CellML and SED-ML files, for which libCellML and LibSEDML support file contents in their respective constructors.

When it comes to our test with a virtual File object, we should convert the file contents of a COMBINE archive to Base64, so that we can put it in a string, in the same way that we have a string version of a CellML and SED-ML file.

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This is even more relevant because of issue #242.

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Check discussion here.