[#657] MMF allocation and API additions for DMI support

[ky-macpherson]

new constructor attribute for MappedMemory
sharedMemory : "/dev/shm"

the attribute string value is the path to the directory where MMFs will be created for segments. On Ubuntu the setting would be /dev/shm

also some public methods are added to the MappedMemory class for SystemC integration

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[ky-macpherson]

Also if the sharedMemory attribute is not set, the class defaults to the prior behavior: segments are allocated in heap memory and not shareable