Multipliable dimarrays

[ggebbie]

This PR is incomplete and does not attempt to pass all tests. It is recommend to not merge until complete.

Features:

• Inputs are streamlined by bundling all information with its central estimate and uncertainty. The Estimate type has this information and it is now used for solutions, first guesses, and observations.
• Recommended for combine to replace solve with the idea that information from two Estimates is combined to make a new Estimate. solve previously required an underdetermined or overdetermined problem to be specified, but now there is limited logic to do this automatically.
• Operations can be performed on arbitrary DimArrays using the MultipliableDimArrays.jl package.
• Consistently uses DimArrays, including for coefficients
• Rules for translating an arbitrary DimArray into an object for linear algebra operations using the DimArray{Number} for vectors and DimArray{DimArray} (i.e., nested types) for matrices
• combine does not require the observational operation to be in linear or matrix form. It currently accepts a Function argument which makes a priori impulse reponse calculations unnecessary.
• UnitfulMatrix wraps a DimArray in order to include units with the numerical values with this option that uses efficient storage of unit information

Future features:

• Units will be included in an extension and are optional
• Next step: go to multipliable-dimarrays branch and run/implement test_dimensional_data with units

[ggebbie]

superceded by PR #59