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improved ViewObj for external objects
improved inline comments
add the external object to the weak pointer list as soon as possible

(even before adding the properties to the external object)
get rid of the obsolete variable assignments_pending in homalgSendBlocking
simplified the assignment of used_pointers in homalgSendBlocking
take CTRL-C into account

homalgCreateStringForExternalCASystem now does neither create the
external object nor SetEval it in the void matrix, rather it passes the
list of pairs of [ external object, void_matrix ] back to homalgSendBlocking