Allow unit-based protocol options to be passed as strings

[lohedges]

This PR enables unit-based protocol options to be passed as strings, e.g:

# These are equivalent.

# Instantiate from types.
p0 = BSS.Protocol.Equilibration(
    timestep=1 * BSS.Units.Time.femtosecond,
    runtime=100 * BSS.Units.Time.picosecond,
    temperature_start=0 * BSS.Units.Temperature.kelvin,
    temperature_end=298 * BSS.Units.Temperature.kelvin,
    pressure=1 * BSS.Units.Pressure.atm,
    restraint="backbone",
    force_constant=100
    * BSS.Units.Energy.kj_per_mol
    / BSS.Units.Length.nanometer**2,
)

# Instantiate from strings.
p1 = BSS.Protocol.Equilibration(
    timestep="1fs",
    runtime="100ps",
    temperature_start="0K",
    temperature_end="298K",
    pressure="1atm",
    restraint="backbone",
    force_constant="100 kJ / (mol nm^2)",
)

This means less typing and makes it easier to instantiate a protocol using settings in a configuration file. (Although none of this is a problem if using nodes.) There are other places in the code where the same approach could be applied, but this is the main point of friction. (I'll update the others as I refactor other parts.)

I've taken the opportunity to add some equality operators to make the testing easier. A lot of the changes are boiler-plate around that.

• I confirm that I have merged the latest version of devel into this branch before issuing this pull request (e.g. by running git pull origin devel): [y]
• I confirm that I have added a test for any new functionality in this pull request: [y]
• I confirm that I have added documentation (e.g. a new tutorial page or detailed guide) for any new functionality in this pull request: [y]
• I confirm that I have permission to release this code under the GPL3 license: [y]

Suggested reviewers:

@chryswoods

[lohedges]

(I clearly took inspiration from the Sire dynamics code.)

[chryswoods]

Tests look good. I'm pleased that sire.u was useful :-)