Resolve "Redesign MpdTpcDigit to BaseTpcDigit"

We need a universal base class for Tpc Digit, to use it in universal tests. This does not change what is now in the codebase.
We will achieve this by having

MpdTpcDigit: public BaseTpcDigit

BaseTpcDigit class will contain information used in universal tests, everything else is in particular implementations

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Closes #146 (closed)