Refactor and update metric calc

Changes:

• using numdifftools package as primary metric calculator (Hessian)

• reimplemented numerical derivatives, though not currently used (will reexamine in future)

• tested three coordinate systems for local metric accuracy against fft metric:

  • \{m_1, m_2\}
  • \{\log_{10} m_1, \log_{10} m_2\}
  • \{\log_{10} m_1, \log_{10} m_2, \chi \}

• Added test coverage for utilities