update environment.yml to specify minimum versions

As opposed to enforcing specific versions, give minima. This avoids some
problems with recent conda.

environment.yml

@@ -3,18 +3,18 @@ channels:
   - conda-forge
   - defaults
 dependencies:
-  - bash-completion=2.11
-  - cmake=3.21.3
-  - fmt=8.1.1
+  - bash-completion>=2.11
+  - cmake>=3.21.3
+  - fmt>=8.1.1
   - compilers
-  - h5py=3.7.0
-  - hdf5=1.12.1
-  - howardhinnant_date=3.0.1
-  - ipykernel=6.19.2 
-  - ninja=1.11.0
-  - nlohmann_json=3.11.2
-  - numpy=1.24.3
-  - python=3.11.3
-  - shellcheck=0.8.0
-  - xtensor-fftw=0.2.5
-  - xtensor=0.24.2
+  - h5py>=3.7.0
+  - hdf5>=1.12.1
+  - howardhinnant_date>=3.0.1
+  - ipykernel>=6.19.2
+  - ninja>=1.11.0
+  - nlohmann_json>=3.11.2
+  - numpy>=1.24.3
+  - python>=3.11.3
+  - shellcheck>=0.8.0
+  - xtensor-fftw>=0.2.5
+  - xtensor>=0.24.2