Can't create orbit

[mileslucas]

Found a small bug that happened where the mean anomaly would not get reduced to a unitless number automatically, which creates a method error for AstroLib.kepler_solver.

orbit = KeplerianOrbit(;
    period=(40.57±0.19)u"yr",
    ecc=0.42±0.009,
    Omega=(318.6±0.6)u"°",
    tp=(1972.12±0.16)u"yr",
    incl=(54.7±0.6)u"°",
    a=(0.154±0.001) * 144.51u"AU",
    omega=(72.6±0.8)u"°"
)
pos = relative_position(orbit, orbit.tp)

Output:

ERROR: MethodError: no method matching kepler_solver(::Quantity{Measurement{Float64}, NoDims, Unitful.FreeUnits{(AU⁻³ᐟ², m³ᐟ²), NoDims, nothing}}, ::Float64)
Closest candidates are:
  kepler_solver(::Real, ::Real) at ~/.julia/packages/AstroLib/Yq5lS/src/kepler_solver.jl:4
Stacktrace:
 [1] compute_true_anomaly(# ...

it gets fixed by throwing a uconvert(u"NoUnits", M) in the compute_true_anomaly method, which should work fine because normal numbers won't be affected by the conversion. I have a PR ready for this.