Merge pull request #238 from brian-team/state_updater_fix

State updater fix
Infer `Variable.is_bool` from dtype instead of explicitly storing/providing it.
Rename `bool` to `boolean` in equations
Add a new preference `default_integer_dtype` and rename `default_scalar_dtype` to `default_float_dtype`.
Allow to use "integer" as the unit specification in equations. Closes #236
Add tests for integer variables and dtypes in general
Add a little note in the documentation about the declaration of dimensionless variables.
Merge pull request #241 from brian-team/int_as_unit

Integer variables in equations
Add very basic test for multiplicative noise
Make sure all symbols have `real=True` to make substitution work. Fixes #242
Merge pull request #243 from brian-team/fix_issue_242

Fix issue 242
Fixes #244
Use parentheses when substituting negative constants in the code to avoid replacing `-C` by `--0.5`, for example.
Correct an outdated code example
[ci skip]
Fix an issue where upsampling could lead to an integer overflow in a long `TimedArray` with a large dt.
Fix the resizing of `DynamicArray1D` and add a couple of tests. Fixes #248
Remove the `allocate_array` function that isn't used anymore.
Start using a specific `Proxy` class instead of `weakref.proxy` for links to other `BrianObject`s.
Don't use weak references in `get_local_namespace`
Add a short document explaining the different reference types.