Add error hint for MethodErrors on LogDensityModel

[penelopeysm]

If a user creates their own AbstractMCMC sampler and passes it something that obeys the LogDensityProblems.jl interface, AbstractMCMC quietly wraps it in a LogDensityModel type which doesn't obey the same interface, which can be mildly confusing. (Well, it did catch me out, so there may be other new users out there too who bumped into it too :))

As this is a conscious decision (cf. #110 (comment)) this PR makes it such that if somebody tries to call LogDensityProblems.jl methods on a LogDensityModel, they get a friendly nudge in the right direction.

MWE:

julia> using LogDensityProblems

julia> using AbstractMCMC: LogDensityModel

julia> struct MyModel end

julia> # In practice one would define more methods, but this is the only one needed for AbstractMCMC to not complain

julia> LogDensityProblems.capabilities(::Type{MyModel}) = LogDensityProblems.LogDensityOrder{0}()

julia> lm = LogDensityModel(MyModel())
LogDensityModel{MyModel}(MyModel())

julia> LogDensityProblems.logdensity(lm, 0)
ERROR: MethodError: no method matching logdensity(::LogDensityModel{MyModel}, ::Int64)
AbstractMCMC.LogDensityModel is a wrapper and does not itself implement the LogDensityProblems.jl interface. To use LogDensityProblems.jl methods, access the inner type with (e.g.) `logdensity(model.logdensity, params)` instead of `logdensity(model, params)`.
Stacktrace:
 [1] top-level scope
   @ REPL[7]:1

[penelopeysm]

(Thanks on both points :))

[devmotion]

Maybe it would be good to add a (very simple) test as well to ensure it works as intended?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.23%. Comparing base (2a77f53) to head (13ebdab).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #146      +/-   ##
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+ Coverage   97.19%   97.23%   +0.04%     
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  Files           8        8              
  Lines         321      326       +5     
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+ Hits          312      317       +5     
  Misses          9        9              

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