Use exceptions rather than assert statements for generator

[maxrjones]

This PR changes two assert statements to exceptions for more informative feedback to the user, to prevent unexpected behavior if asserts are disabled in the interpreter, and to avoid returning a generator of length 0 if input_dim[dim] > ds.dims[dim] but (input_dim[dim] - input_overlap[dim]) > ds.dims[dim] (example below, assuming this is unwanted behavior).

import xbatcher
import xarray as xr
import numpy as np

size = 30
t = np.arange(size)
data = np.random.randn(size, size)
ds = xr.Dataset(
    {"A": xr.DataArray(data, coords={"time": t}, dims=("time", "x"))}
)
bgen_1 = ds.batch.generator(input_dims={'time': 40}, input_overlap={'time': 15})
len(bgen_1)

[codecov]

Codecov Report

Merging #63 (34c4c1a) into main (34ca3f9) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #63   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines          134       136    +2     
  Branches        30        32    +2     
=========================================
+ Hits           134       136    +2     

Impacted Files	Coverage Δ
xbatcher/generators.py	100.00% <100.00%> (ø)

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[jhamman]

Thanks @meghanrjones!