add cabs_singles_Fock() option
resolved the unrecognized key " A' "
add cabs_singles_Fock() selection option in command line
solved a lock issue in Registry
Implement Fock-basis [2]_S in PT2R12::cabs_singles_Fock(), further test is needed
clean up the cabs_singles_Fock() code
Implement Dyall partition [2]_S in PT2R12::cabs_singles_Dyall() with MPQC3_RUNTIME, further test is needed
Followed the old code in PT2R12::cabs_singles_Dyall() to build B matrix, small differences in matrix
fixed the convergence limit, now MPQC3_RUNTIME PT2R12::cabs_singles_Dyall() matches old code
Merge branch 'master' of https://github.com/ValeevGroup/mpqc
Minor updates to pacify icpc
Replaced explicit operator bool with the “safe bool idiom”. icpc complains now about the former.
Merge branch 'master' of https://github.com/ValeevGroup/mpqc
Revert "Merge branch 'master' of https://github.com/ValeevGroup/mpqc"

This reverts commit 563aace1e355e26f4e04360dab4054407299b862, reversing
changes made to 95e44e9c5105fd6f202306dfbb32e927de59beb7.
icpc now compiles.
Reversed 74653c9bf3b4e851e2cb2fa795e7bbe21fe8ee6e … operator bool in Ref<T> just needs to be removed.
Made Ref<T>.(non)null protected, removed most instances of their use (they are only used internally by RefSC classes).
Revert "Made Ref<T>.(non)null protected, removed most instances of their use (they are only used internally by RefSC classes)."

This reverts commit 5ba194cc7cd072f93d5f01039e3630374786db18.
Made Ref<T>.nonnull protected, removed most instances of its use (now only used internally by RefSC classes). Cannot do the same for .null() due to the interaction with RefSCDimension::operator int, etc.
Merge branch 'master' of https://github.com/ValeevGroup/mpqc