Support imposing relationships between parameters as part of a prior

[dvandyk]

Requires resolution to issue #793 first.

Add means to impose relations between parameters as part of a prior component of the analysis.

Envisaged syntax

parameters:
  ublnu::Re{cV} # vector part of ub current
  ublnu::Re{cA} # axial part of ub current
priors:
  - name: WET
    descriptions:
      - { parameter: 'ublnu::Re{cA}', ... }
    relations: 
      ublnu::Re{cVL}: "0.5 * ([[ublnu::Re{cV}]] - [[ublnu::Re{cA}]])"
      ublnu::Re{cVR}: "0.5 * ([[ublnu::Re{cV}]] + [[ublnu::Re{cA}]])"

Possible applications:

• change of basis of WET coefficients
• connecting form factor parameters in HQE