Previously, most dimension mismatch asserts used raw `assert!` and did
not include the mismatching dimensions in the panic message. When using
dynamic matrices, this led to somewhat-opaque panics such as:
```rust
let m1 = DMatrix::<f32>::zeros(2, 3);
let m2 = DMatrix::<f32>::zeros(5, 10);
m1 + m2 // panic: Matrix addition/subtraction dimensions mismatch.
```
This patch adds dimension information in the panic messages wherever
doing so did not add additional bounds checks, mostly by simply changing
`assert!(a == b, ...)` cases to `assert_eq!`. After:
```rust
// panic: assertion failed: `(left == right)`
// left: `(2, 3)`,
// right: `(5, 10)`: Matrix addition/subtraction dimensions mismatch.
```
Note that the `gemv` and `ger` were not updated, as they are called from
within other functions on subset matricies -- e.g., `gemv` is called
from `gemm` which is called from `mul_to` . Including dimension
information in the `gemv` panic messages would be confusing to
`mul` / `mul_to` users, because it would include dimensions of the column
vectors that `gemm` passes to `gemv` rather than of the original `mul`
arguments. A fix would be to add bounds checks to `mul_to`, but that may
have performance and redundancy implications, so is left to another
patch.
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