Add dvs as real to optimization problem

Purpose

DVs should be real before adding to pyOptSparse, otherwise implicit casting might fail and hang the optimization process, as happens with SLSQP described in https://github.com/mdolab/MACH-Aero/issues/96. This was initially caused by a change in pyOptSparse (see this PR and comment).

This PR addresses this issue. Additionally, made a small refactor to reduce multiple raise Error calls.

Ideally, DVGeo DVs should not be allocated by default as complex, but cast as needed. This is however a more fundamental change. This PR is intended to get SLSQP working again quickly, as it has been broken for months, and allow SLSQP MACH-Aero integration tests to be added.

Expected time until merged

ASAP as MACH-Aero and SLSQP do not work past pyOptSparse 2.10

Type of change

[x] Bugfix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (non-backwards-compatible fix or feature)
[ ] Code style update (formatting, renaming)
[x] Refactoring (no functional changes, no API changes)
[ ] Documentation update
[ ] Maintenance update
[ ] Other (please describe)

Testing

Run MACH-Aero tests

Checklist

[x] I have run flake8 and black to make sure the Python code adheres to PEP-8 and is consistently formatted
[ ] I have formatted the Fortran code with fprettify or C/C++ code with clang-format as applicable
[ ] I have run unit and regression tests which pass locally with my changes
[ ] I have added new tests that prove my fix is effective or that my feature works
[ ] I have added necessary documentation

mdolab / pygeo