ENH: improved type support for numpy arrays (#34)

This change improves type support for numpy arrays in two ways:

1. Do not require numpy arrays to be aligned and contiguous. This fixes
an issue with conversion from or to arrays of different int width.

2. Use numpy data types in C interface. Instead of treating numpy array
data as the C types `int64_t` etc., the code now uses `npy_int64` etc.
to match the `NPY_INT64` dtype descriptor.

A related change is to use `order` instead of `nside` for the UNIQ pixel
indexing functions. This is a backward-incompatible change, but the
functions are fairly new and not used much.

.github/workflows/tests.yml

@@ -0,0 +1,29 @@
+name: Tests
+
+on:
+  push:
+    branches:
+    - main
+  pull_request:
+    branches:
+    - main
+
+jobs:
+  tests:
+    name: Tests
+    strategy:
+      fail-fast: false
+      matrix:
+        runs-on:
+          - ubuntu-latest
+          - macos-latest
+          - windows-latest
+        python-version: ['3.7', '3.8', '3.9', '3.10', '3.11', '3.12']
+    runs-on: ${{ matrix.runs-on }}
+    steps:
+    - uses: actions/checkout@v4
+    - uses: actions/setup-python@v5
+      with:
+        python-version: ${{ matrix.python-version }}
+    - run: pip install '.[test]'
+    - run: pytest -v --pyargs healpix