Unify compile-time complex and real builds as run-time options in a single build

**Is your feature request related to a problem? Please describe.**

Currently QMCPACK separates `real` and `complex` functionality at compile-time. Thus, stakeholders need to create two separate executables to test, deploy and develop functionality.

**Describe the solution you'd like**
The long term goal is to unify these real/complex compile-time build options into a single build that can take equivalent runtime options.

**Describe alternatives you've considered**
Since these involve invasive changes in the entire codebase tackling compile-time conditionals e.g. `#ifdef QMC_COMPLEX`, there are a few steps that need to be accounted for. If possible, break down the effort into smaller tasks.

- [x] Explore how many places need to be modified. This [gist](https://gist.github.com/williamfgc/28ffbd23f6888ed89530e3f68c992c93) contains information extracted for `#ifdef QMC_COMPLEX`
- [ ] Initially replace `#ifdef QMC_COMPLEX` by a proper variable at the `qmcapp.cpp` main level and propagate through the `qmcplusplus` namespace. This should not be an invasive and breaking change.
- [ ] Explore more invasive changes: at the template level in QMCPACK internal functionality and CMake's build system configuration level
- [ ] Explore testing options (separate real and complex tests).
- [ ] Allow the runtime option at the top-most level (e.g. configuration xml file header)

**Additional context**
This issue tracks efforts and discussion related to this integration.