Forbid import when tensorflow is not installed (#186)

* Forbid import when tensorflow is not installed

* Update README with conda trickeries

README.md

@@ -50,6 +50,16 @@ pip install cellfinder-napari
 N.B. To speed up cellfinder, you need CUDA & cuDNN installed. Instructions
 [here](https://brainglobe.info/documentation/general/gpu.html).
 
+#### Conda Install
+`cellfinder-core` is available on `conda-forge`, however `tensorflow`, one of it's core dependencies, is not.
+As a result; you _must_ manually install `tensorflow` into your environment - whether you do this before or after `conda install`ing doesn't matter.
+Once you are ready, install `cellfinder-core` into the desired environment via conda;
+```sh
+conda install -c conda-forge cellfinder-core
+```
+
+Please bear in mind that running the `conda install` command above will exit without failing even if `tensorflow` is not present; however `tensorflow` will not be installed as part of this process, and thus `cellfinder-core` will not be usable.
+
 ### Usage
 Before using cellfinder-core, it may be useful to take a look at the
 [paper](https://doi.org/10.1371/journal.pcbi.1009074) which

src/cellfinder_core/__init__.py

@@ -5,6 +5,18 @@
 except PackageNotFoundError as e:
     raise PackageNotFoundError("cellfinder-core package not installed") from e
 
+# If tensorflow is not present, tools cannot be used.
+# Throw an error in this case to prevent invocation of functions.
+try:
+    TF_VERSION = version("tensorflow")
+except PackageNotFoundError as e:
+    raise PackageNotFoundError(
+        f"cellfinder tools cannot be invoked without tensorflow. "
+        f"Please install tensorflow into your environment to use cellfinder tools. "
+        f"For more information, please see "
+        f"https://github.com/brainglobe/brainglobe-meta#readme."
+    ) from e
+
 __author__ = "Adam Tyson, Christian Niedworok, Charly Rousseau"
 __license__ = "BSD-3-Clause"