AlignAndFocusPowderSlim: Only load the 6 banks since everything else assumes we have 6 (#39906)

### Description of work

The prototype algorithm
[AlignAndFocusPowderSlim](https://docs.mantidproject.org/nightly/algorithms/AlignAndFocusPowderSlim-v1.html)
assume we are only loading 6 banks. So for now don't try loading any
other banks.

As started in the docs "hard coded for 6 particular groups". This is to
address the issue of new files having additional banks.

This algorithm is under heavy development and we need it to just work
for testing.

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### To test:

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This should now be able to load newer files like `VULCAN_260351`.

*This does not require release notes* because this is a prototype
algorithm that is now doing what the docs say.

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Author: rosswhitfield

Framework/DataHandling/src/AlignAndFocusPowderSlim.cpp

@@ -335,10 +335,11 @@ void AlignAndFocusPowderSlim::exec() {
   H5::H5File h5file(filename, H5F_ACC_RDONLY, Nexus::H5Util::defaultFileAcc());
   if (itClassEntries != allEntries.end()) {
     this->progress(.17, "Reading events");
-    const std::set<std::string> &classEntries = itClassEntries->second;
+    // const std::set<std::string> &classEntries = itClassEntries->second;
 
     // filter out the diagnostic entries
     std::vector<std::string> bankEntryNames;
+    /*
     {
       const std::regex classRegex("(/entry/)([^/]*)");
       std::smatch groups;
@@ -356,6 +357,12 @@ void AlignAndFocusPowderSlim::exec() {
         }
       }
     }
+    */
+
+    // hard coded for VULCAN 6 banks
+    for (size_t i = 1; i <= NUM_HIST; ++i) {
+      bankEntryNames.push_back("bank" + std::to_string(i) + "_events");
+    }
 
     // each NXevent_data is a step
     const auto num_banks_to_read = bankEntryNames.size();