Add unit class to FlowerMD and allow using real units in Simulation runs

[marjanalbooyeh]

This PR introduces a new Unit class to FlowerMD, which contains commonly used units from unyt package.

With this change, all values with units in FlowerMD are formatted as value * flower.Units.<unit>. For example, a temperature in Kelvin is 300 * flowermd.Units.K.

This PR also makes the following changes to the interface of run functions in Simulation class:

• The variable kT is now temperature , which can handle both kT values (unitless) and temperature values with units.
• The n_steps variable is now called duration, which can be both a number (number of steps) or time length with units.

ToDo:

• Make same changes for pressure and add pressure units

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[marjanalbooyeh]

Couple of issues that we need to look into:

• Build workflow fails due to some mamba env error. This happens only for Ubuntu. I couldn't pinpoint the error, it seems to be __glibc version.
• Some tests are failing. In test_molecules.test_polymer_different_num_mol, looks like there's a key error with mbuild compound. this might be because of recent changes on mbuild side that we need to account for. I'll look into it more closely.
• A new error has showed up in coarse graining tutorials when we want to visualize the coarse grained compounds in jupyter nb. It's coming from the Grits's CG_Compound.visualize() but once I narrowed it down, the error initiates from mbuild clone in method called by grits. This clone method doesn't copy all CG_Compound attributes including atomistic which causes error here.
• We also need to discuss how we want to handle temperature ramps in simulation run functions. Right now, the assumption is that temperature is passed either as a float (i.e. kT) or a unit value (for example 200 * Units.k). But we also support Hoomd ramps that have two kT values in them. We need to manage the temperatures in the Ramp object similarly. My suggestion is to define a Ramp class for flowermd with similar methods for temperature check. Users can only use flowermd's Ramp if they want to use a ramp. Same argument with pressure ramps.