Drop unnecessary dependencies

[jwallwork23]

Not strictly required for #57 but related work.

meshio and tensorboardX are never actually used in the code.

torch is used, but it should be installed for the desired architecture before attempting to install UM2N.

[ddundo]

Since you're assuming that torch is preinstalled maybe it would be useful to have a check like this somewhere:

try:
    import torch
except ImportError:
    raise ImportError(
        "PyTorch is required to use this package. Please install it first."
        " Visit https://pytorch.org/get-started/locally/ for installation instructions."
    )

[jwallwork23]

Since you're assuming that torch is preinstalled maybe it would be useful to have a check like this somewhere:

try:
    import torch
except ImportError:
    raise ImportError(
        "PyTorch is required to use this package. Please install it first."
        " Visit https://pytorch.org/get-started/locally/ for installation instructions."
    )

Good idea, thanks. Addressed in 53694d6. I tried to create a UM2N.torch module that imports torch this way so that this try/except is always hit, but it would've caused widespread changes that probably aren't worth making for the sake of this PR.

[ddundo]

Looks great to me! :)